BERKELEY 

LIBRARY 

UNIVEBSHT  Of 
CALIPOtMIA 


/D6: 


THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 


GIVEN  WITH  LOVE  TO  THE 

OPTOMETRY  LIBRARY 

BY 

MONROE  I.  HIRSCH,  O.D.,  Ph.D. 


ESSENTIALS 


REFRACTION 


Sp  Thomas  G.  Atkinson,  M.  D. 


SS!l 


CHICAGO 
G.  P.  ENGELHARD  <!^  COMPANY 

1914 


/ 


OPTOMETRY 


Copyright  1914. 
By  G.  P.  ENGELHARD  &  CO. 


Prn 


O) 


prefacp:. 

The  motif  and  subjtt't  nuitter  of  this  little  book  is 
slu'eily  ;ni(l  ;.olely  refraction.  It  makes  no  pretense  of 
being  an  exhanstive  treatise  on  optics  or  a  text  book  of 
diseases  of  the  eye.  These  departments  properly  per- 
tain to 'the  eye  specialist,  for  whom  this  book  was  not 
written.  But  nnfortnnately  they  have  drawn  after 
them  the  simpler  practice  of  refraction,  and  it  is  this 
consideration  which  has  led  the  author  to  present  the 
essentials  of  the  latter  subject  in  a  separate  and  dis- 
tinct  form. 

The  science  of  refraction  is  based  upon  a  few  simple, 
well-defined  principles  of  optics,  easily  understood  and 
applied.  It  is  the  purpose  of  this  little  book  to  set 
forth  these  principles  as  clearly  and  concisely  as  pos- 
sible, and  explain  their  application  to  the  successful 
fitting  of  lenses  to  the  eye.  Attention  is  given  solely 
to  those  aspects  of  optics  and  optical  physiolog}'  which 
legitimately  concern  the  refractionist.  It  aims  to  im- 
part, in  terse  and  usable  form,  all  that  is  necessary  to 
the  intelligent  estimation  and  correction  of  refractional 
errors. 

While  not  purporting  to  d(>al  witli  pathological  dis- 
eases of  the  eye,  it  has  been  thought  wise  to  include  a 
chapter  briefly  describing  those  ocular  diseases  wliieli 
are  intimately  connected  with  disturbances  of  vision, 
and  for  whieli,  therefor.',  the  refractionist  is  frequently 
first  consulted.  Thus  warned,  the  refractionist  can,  by 
a  very  reasonable  exercise  of  care,  readily  detect  and 
identify  these  conditions,  and  promptly  refer  tluin  to 
the  oculist  for  appropriate  treatment. 

Special  attention  is  given  to  the  use  of  the  oi.htlial- 
moscope  and  retinoscope.  These  instniments  have  for 
many  years  en'oyed  a   deserved   popularity  among   En- 


ropoati  refractionists  in  tlie  estimation  and  correction 
of  refractional  errors,  and  the  author  believes  they  are 
destined  to  attain  equally  general  favor  in  this  country. 

The  illustrations  are  from  a  series  of  entirely  new 
and  original  drawings,  designed  with  a  view  of  eluci- 
dating those  points  which  the  author's  experience  has 
demonstrated  to  be  essential  points,  most  happily 
demonstrable  by  means  of  diagrams,  but  which,  unfor- 
tunately, are  not  as  a  rule  made  the  subjects  of  illus- 
trations in  text  books  of  refraction. 

The  present  edition  represents  practically  an  entire 
rewriting  of  the  book,  and  the  addition  of  sections  on 
optical  principles  and  hygiene  of  the  eye,  thus  making 
a  complete  manual  on  all  that  pertains  to  the  art  and 
science   of   refraction. 

Chicago,  January  2,   1914. 


CONTENTS. 

Chapter  I            Light  9 

Chapter  II           Optics — Visibility    ID 

Chapter  11 1         Tii  e  Eye 35 

Chapter  lY         Eefkactiox  of  the  Eye.  . .  47 

Chapter  Y           Lenses 57 

Chapter  YI  Accommodation  and 

Convergence    G3 

Chapter  Yll        Retinoscopy   77 

Chapter  YIII      Ophthalmoscopy   97 

Chapter  IX         Correction    of    Hyperme- 

TROPIA 113 

Chapter  X           Correction  of  Myopia 121 

Chapter  XI  Correction  of  Astigma- 
tism    127 

Chai)ti'r  XIT        Practical   Instructions..  141 
Clia})ter  XI 1 1      Strabismus     and     Imbal- 
ance    153 

Chapter  XTY      Asthenopia 1(>9 

Chapter  XY  Diseases  of  the  Eye  Con- 
nected WITH  Disturb- 
ance OF  YisioN 179 

Chapter  XYF       ErrTixo  the  Classes 191 

Chanter  X \'  1 1      11  vcue XE  of  tue  Eye  215 


CHAPTER  I. 

T.IOHT. 

Nature  and  Source  of  Light. 

Lit/hi  is  a  I'oi'iii  (•['  j»liysical  i'iu'i\av,  which,  at-t- 
in^-  upon  the  retina  ol'  the  ('>i\  produces  in  the 
brain  the  sensation  of  vision. 

The  same  word,  Light,  is  used  in  physiology  to 
designate  the  sensation  thus  produced.  This, 
however,  is  not  the  optical  significance  of  the 
term. 

Genekatiox  of  Light. — The  commonest 
modes  of  generation  of  light  are  (1)  the  chemical 
process  of  combustion,  and  (vM  the  nioh'cular 
activity  of  friction. 

SouKCKS. — The  chief  source  ot  light  is  the  sun, 
in  which  probably  chemical  and  molecuhir  activ- 
ity both  take  i^ai't.  The  light  generated  by  the 
sun  is  called  Natural  light.  Other  common 
sources  of  light  are  lamps,  candles,  gas  (cond)us- 
tion),  and  latterly  electric  light  (friction).  Light 
thus  generated  is  called  Artiiicial  light. 

XATrHK.-^The  exact  nature  of  light,  like  that 
of  other  forms  of  physical  energy,  is  not  as  yet 
understood.  We  are  able  to  recognize  and  study 
it  onlv  tlirough  its  effects  upon  the  material 
uu^vlia  which  it  intiuences,  or  whidi  influence  it. 
hi  a  general  way  it  is  understood  to  consist  of  an 
oscillatory  vil)ration  of  the  panicles  of  ether. 

TuANSA.nssiox. — The  utilization  of  light  re- 
quires its  transmission  from  one  point  to  another 


10  REFRACTIOX 

in  si:»ace  through  suitaljle  media.  This  is  accom- 
plished bv  the  vi1)rations  communicating  them- 
selves to  adjoining  particles  of  th(-  >ame  medium 
or  to  those  of  another  medium. 

It  sliould  be  understood,  of  course,  that  actually 
light  vibrations  are  never  communicated  from  one 
medium  to  another,  since  light  vibrations  are  per- 
tinent only  to  ether.  What  actually  happens  is 
that  the  vibrations  are  communicated  from  the? 
ether  in  the  interspaces  of  one  medium  to  the 
ether  in  the  interspaces  of  another  medium.  But 
for  working  convenience  we  say  that  they  are 
communicated  from  one  medium  to  another. 

Transparency  and  Opacity. — Media  or 
bodies  which  permit  of  this  transmission  of  light 
vibrations  through  their  substances  are  called 
transparent.  Those  which  do  not  are  called 
opaque. 

Transparent  and  opaque  are,  of  course,  relative 
terms,  simply  denoting  the  comparative  capacity 
of  different  media  for  transmitting  light  vibra- 
tions. Probably  no  form  of  matter  is  either  ab- 
solutely transparent  or  absolutely  opaque. 

Every  medium  of  greater  density  is  more  or 
less  opaque  as  compared  witli  one  of  less  density. 
Tliat  is  to  say,  the  light  vibrations  of  a  rarer 
medium,  are  never  completely  communicated  to 
a  denser  medium,  some  of  them  ])eing  turned  back 
by  the  denser  into  tlie  rarer  medium. 
.  Transmission  —  Absorption — Eeflectiox.  — 
Of  those  vibrations  which  are  communicated  to  a 


LIGHT  11 

modiniii,  ^nmo  pass  throiiiTli  it  and  arc  re-com- 
imiiiii-att'd  to  anotlier  luediuin;  others  exhaust 
themselves  upon  the  substance  of  the  medium  and 
are  transfoi'ined  into  other  forms  of  energy.  The 
f(U"mc'r  are  said  to  be  tninsniifh'd,  the  hitter  ah- 
sorhciJ.  \)\  the  medium.  Tliose  which  are  turned 
back  by  a  denser  medium  into  a  rarer  medium  arc 
said  to  be  reflected. 

Every  medium — i.  e.,  every  form  of  matter — 
absorbs  light  vibrations  to  a  more  or  less  extent. 

Dynamics  of  Light. 

The  dynamics  of  light  inchule  a  consideration 
of  the  modes  of  franstnisslon ,  relocHy.  force,  and 
effects  upon   matter,  of  its  vibrations. 

Method  of  Transmission. — Light  vi])rations 
al-e  supposed  to  travel  in  the  form  of  waves;  that 
is  to  say,  the  path  of  the  transmitted  vii)ration  is 
marked  by  an  alternate  expansion  and  contrac- 
tion, or,  to  speak  more  correctly,  an  alternate  rari- 
fication  and  condensation,  of  the  medium,  ulti- 
niiilcly  due.  of  course,  to  the  alternate  repulsion 
and  attraction  of  its  atoms. 

Velocity. — Light  vibrations  are  estimated  to 
travel  through  space — i.  e.,  through  luminous 
ether — at  a  speed  of  186,300  miles  per  second. 
This,  however,  is  an  average  estimate,  since  even 
ether  offers  some  resistance  to  their  ])assage,  and 
therefore  their  velocity  progressively  decreasres 
during  transmission. 

The  velocity  of  light  vibrations  varies  directly 


12 


REFRACTION 


as  the  force  of  their   propulsion    (impetus)    and 
inv.crsoly  as  the  density  of  the  medium. 

Force. — Xo  satisfactory  basis  has  been  deter- 
mined for  the  computation  of  the  force  or  impe- 
tus of  light  vibrations.  Hence  in  practical  op- 
tics eacli  source  of  liglit  is  taken  as  an  independ- 
ent   standard,    and    the    second    factor,    viz.,    the 


Illustrating-  the  different   oscillatory  Avave  lengths. 

c()m})arative  densities  of  the  media  through  which 
the  vibrations  pass,  is  the  only  factor  regarded 
in  tlie  determination  of  their  relative  velocity. 
The  denser  the  medium  the  more  slowly  the  vi- 
brations travel. 

OsciLLAToia'  Velocity. — In  addition  to  the  ve- 
locity of  their  transmission,  light  vibrations  have 
a  lateral  or  oscillatory  velocity,  dependent  upon 
tlic  size  of  tlie  vibratoiT  wave — i.  e.,  upon  the 
range  of  repulsion  and  attraction  communicated 
to  tlie  particles  of  ether.  The  larger  the  wave — 
i.  e.,  the  greater  the  range  of  this  repulsion  and 
attraction — tlu*  less  the  oscillatory  velocity. 

Effects  of  Light  Upon  Matter. 

Outride  of  the  already  mentioned  fact  that  light 
\il)rations  propagate  themselves  through  different 
forms  of  matter  at  varying  velocities  and  under 


LIGHT 


i:j 


van'ing  ooiulition.^,  iiotliiiig  is  known  of  any 
pnrol}'  (Ivnaniic  effects  produced  by  them  upon 
objective  media  wliivli  is  of  any  parlicular  \aliie 
in  a  study  of  optics.  1Mie  only  effects  which  con- 
cern optics  are  those  which  light  vibrations  pro- 
duce upon  the  retina  and  are  sul)jectively  inter- 
preted by  the  eye. 

IntfIxsity. — The  relative  transmission-velocity 
of  liglit  vil)rations  produces  an  effect  upon  the 
retina  which  the  l)i'ain  interprets  as  comparative 
intensity  of  light.  There  are,  as  will  presently 
be  seen,  other  conditions  of  the  ocular  apparatus 
itself  which  intluence  intensity,  but  so  far  as  the 
vibrations  themselves  are  concerned,  the  more 
ra[)idly  thev  ai'c  traveling  when  they  strike  the 
retina  the  more  intense  the  sensation  of  light  pro- 
duced,     it   is  for  this  reason,  among  others,  that 


IHustrating-  the  resultant  straight  path  of  tlie  lig:ht 
vibrations,    constituting    the    ray. 

light  received  from  a  near  point  appears  more 
intense  than  that  I'lom  a  fai'  point.  Tliis  cor- 
responds to  the  loudness  of  sound. 

Color. — The  relative  oscillatory  velocity — in 
other  words,  the  relative  wave-length — of  light 
vibrations  is  responsible,  through  its  effects  upon 
the  retina,  for  sensations  of  color.  The  more  rapid 
-these  oscillations — i.  e..  the  shorter  the  waves — • 
the  higher  the  color.  The  shortest  perceptible 
light-waves  gives  the  sensation  of  violet;  the  long- 


1 4  REFRACTION 

est,  that  of  deep  red.     This  corresponds  to  pitch 
in  sound. 

Geometries  of  Light. 

Closely  allied  to  the  dynamics  of  light,  ])ut 
technically  distinct  from  them^  are  its  geometric 
relations,  upon  which  the  whole  system  of  op- 
tics, so  far  as  it  relates  to  refraction,  is  based. 

Linear  Propagatiox. — Foremost  of  these  geo- 
metric postulates  is  the  well-known  axiom  that 
light  yibrations  are  propagated  in  a  straight  line, 
and  cannot  be  made  to  trayel  in  any  other  kind 
of  course.  This  is  what  makes  it  necessary  for 
an  object  to  be  in  our  uninterrupted  line  of  yision 
in  order  that  we  may  see  it. 

Rays. — For  optical  purposes,  therefore,  the  yi- 
brations themselyes  are  not  regarded  as  such.  The 
imaginary  straight  lines  in  which  they  trayel  are 
regarded  as  the  units  of  light,  and  are  called  rays. 
A  combination  of  rays,  representing  the  passage 
of  seyeral  yibrations  of  ditferent  wayc-lengths^  is 
called  a  penciL 

DiyERGEXCE  OF  Eays. — Kays  of  light  leaying 
an  object,  whether  reflected  or  generated  by  the 
object,  are  projected  in  a  diyergent  manner  and 
in  all  ayailable  directions,  and  doubtless  continue 
to  diyerge  as  long  as  they  remain  in  the  same 
medium. 

Infinite  Rays. — At  a  certain  distance  from 
their  origin,  the  angle  of  diyergence  of  those  rays 
which  come  within  our  range  of  yision  is  so  slight 
that  it  is  impossible  to  show  that  they  are  not 


LIGHT 


15 


parallel,  and  for  optical  purposes  they  are  then 
regarded  as  parallel.  Experience  has  shown  this 
distance  to  l)e  six  meters  or  over.  Kays,  there- 
fore, M'liicli  oriuiiiale  six  meters  or  more  from  the 


Showing  how  the  angle  of  reflection  CBP  is  equal 
to    the   angle    of    incidence   ABP. 

observer  are  said  to  come  from  infinity,  are  called 
infinite  rcn/s^  and  are  regarded  as  parallel. 

Finite  I^ays. — Rays  which  proceed  from  an 
object  less  than  six  meters  from  the  observer  are 
called  finite  rays,  and  are  divergent. 

Reflection. — As  previously  stated,  when  rays 
of  light  strike  the  surface  of  a  denser  nuMlium  not 
all  of  them  are  communicated  to  the  new  medium, 
some  of  them  being  turned  back  into  the  rarer 
medium.  These  are  said  to  be  reflected.  The  re- 
flecting power  of  a  medium  is  proportionate  to 
the  smoothness  of  its  surface. 

Angle  of  Incidence  and  Reflection. — The 
angle  which  a  ray  striking  such  a  surface  makes 
witli  the  pci'pondieular  of  tlio  >urface  is  called  the 


16 


REFRACTIOX 


angle  of  incidence.  The  angle  which  the  same 
ray  makes  with  the  same  perpendicular  after  re- 
flection is  called  the  anole  of  reflection. 


Illustrating'   how   a    ray   AB,    upon    entering   a   denser 
medium,   is   bent   toward  the  perpendicular,  as   BC,   and 
iipon   entering  a   rarer  medium   is   bent  away   from   the 
perpendicular,    as    CD. 

Laavs  of  Ekflection. — Eeflection  takes  place 
in  accordance  with  two  geometric  laws,  viz. : 

1.  The  angle  of  incidence  is  equal  to  the  angle 
of  reflection. 

2.  The  incident  and  reflected  rays  are  hoth  in 
the  same  plane,  which  is  perpendicular  to  the 
reflecting  surface. 

Eefractiox. — "When  a  ray  of  light  passes  from 
one  medium  into  another  of  different  density,  if 
the  surface  of  the  medium  into  which  it  passes 
is  perpendicular  to  the  ])nth  of  the  ray  it  con- 
tinues to  travel  in  the  same  straight  line. 

If,  however,  the  surface  of  the  receiving  me- 
dium is  not  perpendicular  to  the  ray,  the  latter, 
upon   entering   it.   is  hent   or   deflected   from   its 


LIGHT  17 


course.  This  bending  of  a  ray  is  called  Refrac- 
tion. If  it  passes  into  a  denser  medium  it  is  bent 
toward  the  perpendicular  of  the  surface;  if  into  a 


pf 


I 
I. 

Illustrates  the  index  of  refraction.  AB  represents 
the  incident  ray  entering-  the  surface  of  the  refracting 
medium;  B  C  the  refracted  ray.  XY  is  the  sine  of 
the  ang-le  XBY  made  by  the  incident  ray  with  the  per- 
pendicular PP'.  X'Y'  is  the  sine  of  the  angle  X'BY' 
made  by  the  refracted  ray  with  the  same  perpendicu- 
lar. The  ratio  between  XY  and  X'Y'  is  the  index  of 
refraction   of  the  two  media. 

rarer  medium  it  is  bent  away  from  that  perpen- 
dicular. 

Index  of  Refraction. — Naturally,  the  rela- 
tion of  the  angle  of  incidence — i.  e.,  the  angle 
which  a  ray  striking  such  a  surface  makes  with 
the  perpendicular — to   the   angle  of  refraction — 


18  REFRACTION 

i.  e.,  the  angle  which  the  same  ray  makes  with 
the  same  perpendicular  after  refraction — is  not 
uniform,  as  in  the  case  of  reflection,  but  varies 
with  the  comparative  densities  of  the  respective 
media. 

For  optical  purposes  we  estimate  the  degree  of 
refraction  by  comparing  the  sine  of  the  angle  of 
incidence  with  the  sine  of  the  angle  of  refrac- 
tion, and  the  ratio  between  these  two  geometric 
quantities  is  called  the  index  of  refraction  of  one 
medium  as  compared  with  the  other. 

For  working  convenience  we  regard  air  as  the 
standard  medium,  and  the  ratio  between  the  sines 
of  the  angles  which  a  ray  of  light  makes  with  the 
perpendicular  before  and  after  passing  from  air 
into  a  given  medium  is  said  to  be  the  index  of 
refraction  of  that  medium.  The  index  is  plus  or 
minus  according  as  the  ratio  is  in  favor  of  or  at 
the  expense  of  the  angle  of  refraction. 

Example :  A  ray  of  light  passing  from  air 
into  water  and  refracted  by  the  water.  The  sine 
of  the  angle  of  incidence  is  1.333  times  greater 
than  the  sine  of  the  angle  of  refraction.  There- 
fore the  index  of  refraction  of  water  is  said  to 
be  -f  1.333. 

N.  B. — In  optics,  so  far  as  they  relate  to  re- 
fraction of  eyes,  we  never  have  to  do  with  any  re- 
fracting medium  of  less  density  than  air.  Hence 
the  index  of  refraction  of  those  media  which  con- 
cern us  is  always  plus,  and  no  attention  need  be 
paid  to  minus  indices. 


CHAPTER  II. 
OPTICS— VISIBILITY. 

The  visibility  of  an  object  is  clue  to  (1)  its 
capacity  for  reflecting  part,  but  not  all,  of  the 
rays  of  light  which  strike  its  surface,  and  (2) 
its  ability  to  change  the  dynamic  qualities  of 
those  rays  which  it  reflects.  An  object  which 
either  transmits,  or  absorbs,  or  reflects  all  of  the 
rays  which  it  receives  is  not  visible  as  an  object. 


Showing-  how  (vision  being-  wholly  due  to  reflected 
rays)  an  object  which  transmits  all  the  rays  and  re- 
flects  none   of   them   is   invisible   to   the  eye. 

An  object  which  perfectly  transmits  all  of  the 
rays  which  strike  its  surface  is  absolutely  invis- 
ibio. 

An  object  which  completely  absorbs  all  of  the 
rays  which  reach  it  is  seen  simply  as  an  area  of 
shadow. 


20 


REFRACTION 


An  object  which  uniformly  reflects  all  of  the 
rays  which  strike  its  surface  is  seen  sheerly  as  an 
area  of  light,  similar  in  all  respects  to  the  source 
of  its  illumination. 


MIRROR 


Showing  how  an  object  like  a  mirror,  which  reflects 
all  the  rays  is  itself  invisible  but  appears  to  the  eye 
in  all  respects  as  the  original  source  of  light. 

Those  substances  which  we  usually  regard  as 
being  quite  transparent  (air,  crystal  glass,  water, 
etc.)  perfectly  transmit  some  of  the  rays  and 
uniformly  reflect  others,  so  that  we  both  "look 
through  them'^  and  also  see  them  as  an  area  of 
pure  light. 


OPTICS — VISIBILITY 


21 


An  object  which  is  visible  in  detail  absorbs 
part  of  the  light  rays  which  strike  its  surface 
and  reflects  others,  according  to  the  various  form 
and  character  of  its  surface.     Those  rays  which 


Showing-  how  an  object  which  transmits  some  rays 
and  reflects  some  is  visible  to  the  eye  as  an  individual 
image. 

it  reflects  are  changed  in  velocity,  wave-length, 
etc.,  also  in  accordance  with  the  varied  surface 
of  the  object^  and  effects  are  produced  upon  the 
retina  corresponding  to  these  changes.  The  net 
sum  of  these  effects  constitutes  what  is  known 
as  the  retinal  image,  from  which  the  brain  judges 
of  the  identity  of  the  object. 

The  more  nicely  balanced  the  absorption  and 
reflection  of  light,  the  more  clearly  the  object  is 


22  REFRACTION 

seen.  In  ordinary  sunlight  there  is  too  much 
reflection;  this  is  why  we  see  things  in  much 
clearer  detail  just  after  sun-down. 

Eeflected  Image. — All  retinal  images  are 
tliereforo  reflected  images.  However,  when  the 
light  is  reflected  directly  from  the  object  to  the 
retina  we  say  that  we  see  the  object  itself.  It  is 
manifest  that  an  object  can  be  thus  seen  only  when 
it  is  in  an  uninterrupted  straight  line  with  the 
retina.  When  the  light  from  an  object  is  inter- 
cepted by  another  surface,  and  by  it  reflected 
to  the  retina  we  speak  of  the  retinal  image  being 
a  reflected  image.  By  this  means  an  object  may 
become  visible  which  is  not  in  the  direct  line  of 
vision. 

Reflection  of  Light. 

A  body  with  a  highly  and  imiformly  polished 
surface,  which  does  not  transmit  light,  reflects 
practically  all  of  the  rays  which  strike  its  sur- 
face. 

When  the  light  reflected  by  such  a  surface 
comes  from  a  source  of  pure  light,  it  is  seen,  as 
already  stated,  as  an  area  of  pure  light,  similar 
to  the  source  of  illumination. 

AVhen,  on  the  other  hand,  the  light  so  reflected 
reaches  the  polished  surface  from  another  object, 
it  is  reflected  in  all  respects  in  the  same  condi- 
tion as  it  was  received  from  the  original  object, 
and  makes  precisely  the  same  effect  upon  the 
retina    as   thougli   roeeived   by   the   eye  from   tlie 


OPTICS — VISIBILITY 


23 


original  object.     In  other  words,  the  mirror  gives 
a'  reflected  image  of  the  object. 

Projection. — Since  light  always  travels  in  a 
straight  line,  the  brain  is  only  able  to  project 
light  rays — that  is,  to  refer  them  to  an  origin,  in 
a  straight  line.  Hence  the  image  of  an  object 
seen  in  a  mirror  does  not  appear  to  the  brain  to 


APPARENT    POSITION    OF    LIGHT 


Illustrating    the    apparent    position     of    a    reflected 
image. 

he  located  at  the  original  object  but  on  a  straight 
line  projected  from  the  retina  through  the  point 
at  which  the  rays  strike  the  mirror. 

But  the  brain  judges  of  distance  by  the  dy- 
namic qualities  of  the  rays  which  reach  the  retina, 
and  these  have-not  been  changed  by  the  process 
of  reflection.  'Fherefore  the  distance  at  which 
the  image  a{)}X3ars  to  be  located  is  the  distance 
which   tlio   rays  have   actually   traveled— namely. 


24 


REFRACTION 


the  distance  from  the  object  to  the  mirror  plus 
the  distance  from  the  mirror  to  the  eye. 

The   apparent   location  of  the  reflected  image 
from  a  plane  mirror,  therefore,  is  in  a  straight 


Showing  how  parallel  rays  are  focused  by  a  con- 
cave mirror  at  a  point  F,  midway  between  the  surface 
S  and  the  optical  centre  C.  The  distance  SF  is  the 
focal  length   of  the  mirror. 

line  from  the  eye  through  the  reflection  point  on 
the  mirror,  as  far  beyond  the  mirror  as  the  ob- 
ject is  from  the  reflection  point. 

Concave  Reflection  of  Light. 

A  concave  surface  is  to  be  regarded  as  made 
up  of  a  number  of  plane  surfaces  inclined  to- 
ward each  other. 

The  optical  center  of  a  concave  mirror  is  the 


OPTICS — VISIBILITY 


25 


center  of  the  sphere  of  whiclj  the  concave  surface 
is  a  segment. 

Principal    Focus — Focal   Length. — Parallel 
rays,  falling  upon  a  concave  mirror,  are  reflected 


Illustrating  conjugate  foci,  F  and'  F'.  If  tlie  light 
be  at  F  the  reflected  image  will  focus  at  F',  but  if  the 
light  be  at  F'  the  image  will  focus  at  F. 


as  convergent  rays  which  meet  at  a  point  on  the 
axis  midway  between  the  surface  of  the  mirror 
and  the  optical  center.  This  point  is  called  the 
principal  focus  of  the  mirror,  and  the  distance 
from  the  surface  to  this  point  is  called  the  focal 
length  of  the  mirror. 

Con/ersely  it  follows  that  rays  originating  at 


26  REFRACTION 

the  focal  point  of  a  concave  mirror  are  reflected 
as  parallel  rays  from  the  surface  of  the  mirror. 

It  also  follows  that  rays  which  originate  at  the 
optical  center  of  a  concave  mirror  are  reflected 
back  from  the  mirror  in  the  same  lines,  and  the 
object  is  its  own  image. 

Com  JUGATE  Foci. — If  the  origin  of  the  rays  be 
at  a  point  within  the  optical  center  (but  not  with- 
in the  principal  focus)  the  reflected  rays  will  con- 
verge at  a  point  an  equal  angular  distance  witli- 


Illustrating  a  virtual  focus  F.  The  light  at  L  pro- 
jected on  a  concave  mirror  will  appear  from  any  point 
between  A  and   B  to  be  at  F. 

out  the  center.  N"ow  if  the  point  of  convergence 
outside  the  optical  center  be  made  the  point  of 
origin  the  former  point  of  origin  inside  the  op- 
tical center  becomes  the  point  of  convergence  of 
the  reflected  rays.  These  two  points  have  there- 
fore a  reciprocal  relation  to  each  other,  and  are 
called  conjugate  foci. 

Virtual  Focus^ — Virtual  Image. — If  the  rays 
originate  at  a  point  inside  the  principal  focus  of 
a  concave  mirror,  by  the  laws  of  reflection  the 


OPTICS — VISIBILITY 


27 


rays  are  reflected  from  the  surface  as  divergent 
rays,  and  never  meet.  As  explained  above,  an 
eye  in  the  path  of  these  divergent  rays  will  in- 
terpret them  as  coming  from  a  point  in  a  straight 


^^. 


--^d- 


Illustrating  how  a  real  reflected  image  becomes  in- 
verted. 

line  from  the  eye  through  the  point  of  reflection 
on  the  mirror,  and  as  far  behind  the  mirror  as 
the  originating  point  is  from  the  surface. 

The'point  at  which  tlie  image  thus  appears  to 
be  located  is  called  the  virtml  focus,  and  the 
image  thus  seen  a  virtual  image. 

A  concave  mirror  therefore  gives.two  kinds  o± 
image  or  no  image  at  all,  according  to  the  loca- 
tion of  the  object. 

If  within  the  principal  focus,  it  gives  an  erect 
virtual  image,  because  the  rays  are  reflected  di- 
vergently and  never  meet. 

If  at  the  optical  center,  there  is  no  image  at 
all,  because  the  rays  are  reflected  so  as  to  make 
the  object  its  own  image. 


28 


REFRACTION 


If  outside  the   optical   center^   it  gives   a   real 
inverted  image,  because  the  rays  are  reflected  so 
as  to  meet  at  a  point  within  the  center,  and  have 
therefore  crossed  before  they  reach  the  eye. 
Refraction  of  Light. 

Every  body  or  substance  which  transmits  light 
exercises  more  or  less  power  of  refraction — i.  e., 
it  deflects  the  ray  more  or  less  from  its  original 
course,   provided   the   ray    strikes   its    surface   at 


'/ 


'> 


^y>^ 


Illustrating   how   a  virtual    reflected   image   remains 
erect. 


other  than  a  perpendicular  angle.  By  this  means, 
as  well  as  by  reflection,  objects  may  be  rendered 
visible  which  are  not  in  an  uninterrupted  straight 
line  with  the  eye. 

A  familiar  example  of  this  may  be  experienced 
by  placing  a  coin  in  the  bottom  of  an  empty  bowl 
and  moving  away  until  the  side  of  the  bowl  just 
hides  the  coin  from  view,  then  having  someone 
pour  water  into  the  bowl,  whereupon  the  coin 
will  again  come  into  sight,  owing  to  the  refrac- 


OPTICS — VISIBILITY 


29 


tion  by  the  water  of  the  rays  proceeding  from  the 
surface  of  the  coin. 

Degrees  of  Eefk action. — The  degree  of  re- 
fraction exercised  by  a  medium  depends  upon  (1) 
its  relative  density,  and  (2)  the  angle  at  which 
the  ray  strikes  its  surface. 

Spherical  Refraction, 
Principal  Axis. — When  the  surface  of  the  re- 
fracting medium  is  spherical — i.  e.,  the  segment 


INCIDENT    RAY 


AXIAL   n  A  V 


INCIDENT    RAY 


Illustrating  the  principal  focal  point,  F,  and  the 
principal  focal  distance,  BF.  in  convex  refraction. 
Showing'  point  where  the  incident  rays  meet  the  axial 
ray  after  passing  into  a  denser  medium.  AB  axial 
ray,  BP,  focal  distance.  F,  focal  point,  where  rays 
actually  meet  (positive).  Rule  does  not  apply  where 
the  incident  rays  enter  the  medium  very  far  from  the 
axial   ray. 

of  a  sphere — it  is  mathematically  apparent  that 
one,  and  only  one,  of  a  group  of  parallel  or  di- 
vergent rays  which  enter  it  will  do  so  at  right 
angles  to  its  surface.  Ths  one  ray  will  continue 
its  course  through  the  new  medium  in  the  same 


30  REFRACTION 

straight  line  it  had  before.     It  is  called  the  prin- 
cipal ray,  and  its  course  the  principal  axis. 

Principal  Focal  Point  and  Distance. — 
AMien  the  surface  of  the  medium  is  convex,  the 
rest  of  the  rays,  being  bent  toward  the  perpendicu- 
lar of  the  surface,  will,  if  continued  through  the 


inustrating  the  principal  focal  point  and  principal 
focal  distance,  BF,  in  concave  refraction.  Showing 
point  where  the  incident  rays  are  projected  backward 
so  as  apparently  to  meet  the  axial  ray  at  F.  AB, 
axial  ray;  Bt,  focal  distance;  F,  focal  point  where 
rays  appear  to   meet    (negative). 

medium  long  enough,  meet  at  a  point  on  the  prin- 
cipal axis.  When  the  surface  of  the  medium  is 
concave,  the  rest  of  the  rays,  being  bent  to- 
ward the  perpendicular^  are  diverged,  and  will 
never  meet,  but  the  eye,  receiving  these  rays,  pro- 
jects them  to  an  imaginary  point  outside  the  me- 
dium at  which  they  would  geometrically  meet. 
This  point  in  each  case,  where  the  rays  meet  or 
seem  to  me,  is  called  the  principal  focal  point, 
and  the  distance  between  it  and  the  refracting 
surface  is  called  the  principal  focal  distance. 


OPTICS VISIBILITY  31 

Positive  and  Negative. — In  convex  refraction 
the  principal  focal  pointy  where  the  refracted  rays 
actually  meet,  is  said  to  be  positive;  in  concave 
refraction,  where  they  are  projected  to  meet,  it 
is  said  to  be  negative. 

Spherical  Abberation. — If,  however,  we  trace 
the  course  of  rays  which  are  very  oblique  to  tlie 


Illustrating'  spherical  aberration.  Showing  that 
when  the  incident  rays  enter  the  denser  medium  too 
far   from   the  axial   rays  they   do   not   reach   a   focus. 

axial  ray,  or  which  strike  the  refracting  surface 
far  from  its  axis,  we  find  tliat  they  do  not  meet 
the  axial  ray  all  at  the  same  point.  This  lack  of 
exact  reunion  of  all  the  rays  is  known  as  spher- 
ical ahheration,  a  very  annoying  fault  in  optical 
instruments  designed  for  very  accurate  work. 
The  eye  has  this  fault  to  a  small  degree,  but  not 
enough  to  interfere  with  vision. 

Principal   Posterior   Focus. — Rays   emanat- 
ing from  a  point  at  an  infinite  distance  from  the 


32 


REFRACTION 


refracting  surface  strike  the  surface  with  so  little 
divergence  as  to  be  practically  parallel  to  each 
other,  and  if  the  refracting  inedium  be  denser, 
they  are  rendered  convergent  by  refraction.  The 
point  where  these  refracted  rays  met  is  called 
the  principal  posterior  focus,  and  the  distance  be- 
tween it  and  the  refracting  surface  the  principal 
posterior  distance. 

Principal    Anterior    Focus. — At    a    certain 
point  of  nearness  of  the  luminous  point  to  the  re- 


Showing-  the  formation  of  an  inverted  image  by  rays 
passing-   through    a   convex   spherical    medium. 

fracting  surface,  the  rays  are  so  divergent  that 
the  refracting  medium  can  no  longer  render  them 
convergent,  but  only  parallel.  These  refracted 
rays  will  therefore  never  meet,  and  the  point  of 
their  reunion  is  at  infinity.  The  point  from  which 
such  rays  proceed  is  called  the  principal  anterior 
focus,  and  its  distance  from  the  refracting  sur- 
face the  anterior  focal  length. 

Formation  of  Images. — From  every  point  of 
an  object  there  proceeds  one  ray  which  is  not  bent 
from  its  course  by  a  convex  refracting  medium — 


OPTICS — VISIBILITY  33 

viz.^  tlu-  rfT>  Aiiich  strikes  the  refracting  surface 
perpendicular! }'.  Such  rays  arc  called  rays  of 
direction.  These  rays  of  direction  meet  and  in- 
tersect at  the  center  of  curvature  of  the  surface, 
or  the  optic  center.  All  other  rays  proceeding 
from  the  various  points  of  the  object  are  refract- 
ed so  as  to  meet  their  respective  axial  rays  at 
the  principal  posterior  focus.  But  this  posterior 
focuSj  as  we  liave  seen,  is  further  from  the  re- 
fracting surface  than  the  length  of  the  radius, 
therefore  the  focused  image  of  the  various  points 
of  the  object  are  formed  beyond  the  point  of  in- 
tersection of  the  axial  rays,  hence  the  image  of 
the  object  is  an  inverted  one. 

Circles  of  Diffusion. — An  image  is  clearly 
defined  only  at  the  plane  of  the  focal  reunion  of 
the  rays.  In,  any  plane  anterior  to  this  the  rays 
have  not  yet  united;  and  posterior  to  this,  the 
rays  have  united,  crossed,  and  again  diverged. 
A  screen  placed  either  in  front  of  or  behind  the 
plane  of  focal  reunion  therefore  receives  a  blurred 
image,  because  every  point  of  the  object  is  repre- 
sented^ not  by  a  corresponding  single  point,  but 
by  a  circle  of  diffused  rays.  The  size  of  these 
circles  increases  as  the  screen  is  moved  forward 
or  backward.  As  we  shall  presently  see,  this  is 
the  principle  of  spherical  errors  of  refraction  in 
the  eve. 


CHAPTER  III 

THE  EYE. 

For  the  purposes  of  this  work  the  eye  need  be 
considered  only  in  its  capacity  as  an  optical  in- 
stnmiont.  and  in  so  far  as  its  structure  has  to 


Rectus 


Nerve 


Rectus  Muscle"^^^  Sc^ev* 

Illustrating'   the    anatomy    of   the   eye. 

do   with   the   phenomenon   of   refraction   and   its 
accessory  functions. 

The  Anatomic  Construction  of  the  Eye. 
The  i'ljcbaU  is  formed  of  the  segments  of  two 
hollow  spheres^  of  different  size  and  convexity, 
the  smaller  and  more  convex  of  which  is  set  into 
the  larger  and  less  convex  as  a  crystal  watch 
glass  is  let  into  its  case. 


36  REFRACTION 

The  Cornea. — The  larger  spherical  segment 
consists  externally  of  the  sclera,  which  forms  five- 
sixths  of  the  entire  eyeball,  including  all  of  the 
posterior  portion  and  that  part  which  is  inside 
the  bony  orbit.  Into  the  anterior  portion  of  this 
segment  is  inserted  the  smaller  segment,  the 
cornea. 

The  Iris. — At  the  posterior  part  of  the  cornea, 
where  it  is  set  into  the  sclera,  inside  the  globe, 
is  suspended  the  circular  curtain,  called  the  iris, 
which  forms  the  pupil  of  the  eye.  It  is  the  pig- 
mentation of  the  iris  which  gives  color  to  the 
eye. 

The  Pupil. — In  the  center  of  the  iris  is  a  cir- 
cular aperture  through  which  light  passes  in  and 
out  of  the  eye.  This  aperture  is  called  the  pupil. 
To  the  observer  under  ordinary  conditions  the 
pupil  appears  black  because,  as  we  shall  presently 
see,  it  is  impossible  for  the  observers  eye,  under 
ordinary  conditions,  to  receive  and  focus  any  of 
the  rays  proceeding  from  another's  eye. 

The  iris  is  furnished  with  two  sets  of  muscles, 
one  running  circularly  or  concentrically,  the  other 
in  a  radiating  direction.  Contraction  of  the  first 
group  draws  the  iris  centripetally  inward,  lessen- 
ing the  size  of  the  pupil ;  contraction  of  the  latter 
group  draws  the  curtain  centrifugally  outward, 
increasing  the  size  of  the  pupil. 

The  Lens. — Immediately  behind  the  iris  is  the 
crystalline  lens,  a  double  convex  lens  whose  pos- 
terior surface  is  more  convex  than  its  anterior, 


THE    EYE  37 

with   an   elastic   capsule,   and   filled   with   trans- 
parent, colorless  fibres. 

The  Chambers. — The  space  between  the  cornea 
and  the  iris  is  called  the  anterior  chamber,  and 
that  between  the  iris  and  the  lens  the  posterior 
chamber.  (Some  anatomists  deny  the  existence 
of  a  posterior  chamber.) 

The  Hujmors. — These  chambers  communicate 
with  each  other,  and  are  filled  with  a  transparent, 
colorless  fluid  of  a  slightly  greater  density  than 
water,  called  the  aqueous  humor.  The  remainder 
of  the  interior  of  the  sphere,  comprising  four- 
fifths  of  the  entire  globe,  is  filled  with  a  trans- 
parent, jelly-like  substance,  of  about  the  same 
refractive  density  as  glass,  called  the  vitreous 
humor. 

The  Eetina. — Spread  out  upon  the  internal 
posterior  surface  of  the  ball  is  the  retina,  the 
sensitive  membrane  which  receives  the  rays  of 
light,  after  their  passage  through  the  foregoing 
structures,  as  an  inverted  image  of  the  object 
from  which  the  rays  proceeded. 

The  eye  may,  therefore,  be  regarded  as  a  cam- 
era, of  which  the  cornea,  crystalline,  and  vitreous 
are  the  lenses,  the  iris  the  shutters,  and  the  retina 
the  sensitive  plate.    The  perception  and  interpre^ 
tation  of  the  image  are  functions  of  the  brain,  and^ 
belong  to  a  study  of  physiology  and  neurology. 

The  Ciliary. — Around  the  crystalline  lens  is 
the  circular  muscle  called  the  ciliary  muscle,  and 
around  the  extremities  of  the  lens  are  the  sus- 


38  REFBACTION 

pensorj  ligaments,  which  limit  the  convexity  of 
the  lens.  These  ligaments  are  held  in  place  by 
the  choroid,  the  internal  lining  coat  of  the  sclera. 
When  the  ciliary  muscle  contracts,  it  draws  the 
choroid  forward  and  releases  the  suspensory  liga- 
ments, whereupon  the  elasticity  of  the  lens  changes 
its  shape  to  one  of  greater  convexity.  This  has 
an  important  influence  upon  the  refraction  of  the 
eye,  as  will  presently  be  seen. 

The  Yellow  Spot. — Exactly  in  the  center  of 
the  retina,  corresponding  to  the  visual  axis  of  the 
eye,  is  a  small  vascular  spot,  called  the  macula 
lutea  or  yellow  spot.  This  is  the  spot  where  the 
sense  of  vision  is  most  perfect,  and  forms  the 
center  of  the  focusing  system.  In  the  center  of 
the  yellow  spot  is  a  minute  depression,  called  the 
fovea  centralis,  which  is  the  most  sensitive  point 
in  this  sensitive  spot. 

The  Disc. — About  an  eighth  of  an  inch  to  the 
inner  side  of  the  yellow  spot  is  the  place  where 
the  optic  nerve  enters  the  retina.  This  is  marked 
by  a  slight  protuberance,  and  as  it  is  entirely 
devoid  of  sensitiveness  it  is  commonly  called  the 
blind  spot.  In  optical  language  it  is  also  called 
the  disc  of  the  eye,  and  the  fact  of  its  non- sensi- 
tiveness is  taken  advantage  of  in  oplithalmoscopic 
examination  to  focus  the  light  upon  this  point. 

Axes  and  Points. — There  are  various  axes  of 
the  eye — i.  e.,  imaginary  lines  drawn  through  the 
eyeball  in  different  directions,  and  various  points 
situated  at  different  sites  on  the  axes,  which  are 


THE    EYE  39 

important  in  the  study  of  the  optics  of  the  eye, 
and  they  should  be  carefully  studied  and  thor- 
oughly comprehended  by  the  practitioner  of 
optics. 

Since  the  eyeball  consists  of  segments  of  two 
different  sized  spheres,  there  are  two  spherical 
systems  for  which  to  estimate  these  axes  and 
cardinal  points.  However,  the  horizontal  axes 
of  the  two  systems  are  identical,  and  the  cardinal 
points  of  both  systems  are  situated  on  this  hori- 
zontal axis.  Furthermore,  the  respective  points 
for  the  two  systems  are  found  to  be  so  near  to- 
gether that  for  practical  purposes  they  are  re- 
garded as  being  identical. 

There  are,  then^  really  six  cardinal  points  of 
the  eye,  but  practically  only  three.  1.  The  'prin- 
cipal pointy  situated  on  the  horizontal  axis  two 
mm.  behind  the  cornea.  2.  The  nodal  point,  situ- 
ated on  the  same  axis  seven  mm.  behind  the  cor- 
nea. 3.  The  principal  focus,  situated  on  the  same 
axis  where  it  cuts  the  retina. 

The  principal  point  is  the  point  where  the  hori- 
zontal axis  is  cut  by  the  mean  of  the  refracting 
planes  (in  this  case  the  mean  of  the  curvature 
of  the  eye). 

The  nodal  point  is  the  center  of  the  refracting 
system  of  the  eye,  corresponding  to  the  optical 
center  of  a  sphere  already  described,  and  the  rays 
which  pass  through  this  point  are  not  refracted, 
being  rays  of  direction.  A  luminous  point  placed 
above  the  ])rincipal  axis  of  the  eye  forms  its  image 


40 


REFRACTION 


on  the  retina  below  that  axis,  and  vice  versa,  hence 
the  retina  gets  an  inverted  image  of  the  object. 


Showing  that  the  further  forward  the  nodal  point 
is  (as  in  myopia)  the  larger  the  size  of  the  inverted 
image  on   the  retina. 

The  principal  rays  from  these  various  luminous 
points  which  make  up  the  object  cross  at  the  nodal 
point. 

In  the  eye  whose  refraction  is  normal  the  nodal 
point  is  about  7  mm.  behind  the  cornea,  but  its 


Showing  that  the  further  back  the  nodal  point  is 
(as  in  hypermetropia)  the  smaller  the  size  of  the  in- 
verted image  on  the  retina. 

location  of  course  varies  with  the  curvature  of 
the  eye.  It  will  readily  be  seen  that  the  further 
forward  the  nodal  point  is,  where  the  principal 
rays  cross,  the  larger  ^nll  be  the  inverted  image 
on  the  retina;  the  further  back  it  is,  the  smaller 
the  image. 


THE    EYE 


41 


The  principal  focus  is  that  point  on  the  prin- 
cipal (horizontal)  axis  where  the  rays  refracted 
by  the  combined  systems  of  the  eye  are  brought 
to  a  focus.  In  the  normal  eye  it  is  of  course 
located  at  the  retina. 

The  axes  of  the  eye,  with  which  refraction  is 
concerned,  are  (1)  the  principal  axis,  (2)  the  op- 
tic axis,  and  (3)  the  visual  axis. 

The  principal  axis,  as  already  indicated,  is  an 


Showing-   the    principal   points   of   the   eye. 

imaginary  line  drawn  through  the  center  of  the 
eyeball  dividing  it  horizontally  into  two  equal 
parts. 

The  optic  axis  is  an  imaginary  line  drawn 
through  the  center  of  the  cornea  and  the  nodal 
point  to  the  inner  side  of  the  yellow  spot.  (This 
is  practically  identical  with  the  principal  axis.) 

The  visual  axis  is  an  imaginary  line  drawn 
from  the  object  looked  at  through  the  nodal  point 
to  the  yellow  spot. 


42 


REFRACTION 


In  addition  to  the  above  axes  and  cardinal 
points,  there  are  the  axes  of  rotation  of  the  eye- 
ball, imaginary  lines  around  \\liich  the  eyeball 
rotates  by  means  of  the  recti  and  oblique  muscles 
(these  axes  are  vertical,  horizontal  and  oblique), 
and  the  point  of  the  center  of  rotation. 


inustratinpT  the  necessity  for  a  larger  dimension  of 
the  object,  the  further  away  it  is,  in  order  to  con- 
form  to   the   visual   angle. 

Angle  Alpha. — The  angle  formed  at  the  nodal 
point  by  the  optic  and  visual  axes  is  called  the 
angle  alpha.     In  a  normal  eye  it  is  about  5°. 

Angle  Gamma. — The  angle  made  by  the  visual 
axis  and  a  line  drawn  from  the  object  through 
the  center  of  rotation  is  called  the  angle  gamma. 

The  Visual  Angle. — The  angle  made  by  two 
lines  drawn  from  the  extreme  boundaries  of  the 
object  looked  at  tlirough  the  nodal  point  is  called 
tlie  visual  angle.  The  minimum  size  of  this  angle 
in  order  that  the  brain  may  interpret  the  image 
is  1°.  That  is  to  say,  two  luminous  points  sep- 
arated by  an  angular  distance  of  less  than  1° 
would  be  perceived  by  the  brain  as  only  one 
luminous  point. 

Acuity  of  Vision. — The  ability  to  distinguish 


THE    EYE  43 

two  separate  luminous  points  at  a  given  visual 
angle  is^  of  course,  ultimately  a  function  of  the 
brain,  and  has  nothing  to  do  with  refraction.  It 
is  known  as  acuity  of  vision.  It  is,  however, 
habitually  made  use  of  in  testing  refraction;  for 
if  a  patient  in  whom  there  is  no  reason  to  sus- 
pect any  defect  of  the  brain  is  unable  to  obtain 
a  clear  image  of  an  object  at  a  distance  corre- 
sponding to  the  normal  minimum  \isual  angle,  it 
must  be  because  the  condition  of  his  refraction 
is  such  tliat  the  rays  from  that  object  do  not  fall 
upon  liis  retina  at  the  proper  visual  angle.  Any 
correction,  therefore,  which  enables  him  to  dis- 
tinguish such  an  object  at  the  minimum  angular 
distance  has  manifestly  rendered  liis  refraction 
normal.     The  most  familiar  and  useful  applica- 


Illustrates  the  construction  of  the  test  type  to  con- 
form to  the  visual  ang-le  at  a  g-iven  distance.  This 
angle  must  not  be  less  than   1°. 

tion  of  one  to  the  other  is  the  test-type  cards  that 
are  employed  to  test  distant  vision. 

Test  Types. — These  types  are  constructed  on 
the  principle  of  the  visual  angle.  On  these  test 
cards  each  letter  is  so  constructed  that  at  its  proper 
distance  (e.  g ,  ^o.  G  type  at  G  meters)  the  mini- 
mum distance  between  its  parts  is  not  less  than  1°. 


44 


EEFRACTION 


Muscles  of  the  Eye. 

ExTRixsic  Muscles. — In  addition  to  the  ciliary 
muscle  and  those  of  the  iris,  which  are  called  the 
intrinsic  muscles,  the  eyeball  is  furnished  with 
four  pairs  of  quite  good  sized  muscles,  attached 


Showing-  the  muscles  of  the  eye  in  situ. 

to  the  outer  coat,  which  are  called  the  extrinsic 
muscles,  and  which  serve  to  rotate  the  eyeball  on 
different  axes  and  in  different  directions.  Each 
eyeball  has  one  of  these  six  pairs,  as  follows: 

Siiperior  Rectus,  attached  to  the  upper  part  of 
the  eyeball/ which  rotates  it  upward. 

Inferior  Rectus,  attached  to  the  lower  part  of 
the  eyeball,  which  rotates  it  do^vnward. 

Internal  Rectus,  attached  to  the  inner  side  of 
the  eyeball,  which  rotates  it  horizontally  inward. 


THE    EYE  45 

External  Rectus,  attached  to  the  outer  side  of 
the  ball^  which  rotates  it  horizontally  outward. 

Superior  Ohlique,  attached  to  the  outer  and 
upper  side  of  the  ball,  which  rotates  it  outward 
and  upward. 

Inferior  Ohlique,  attached  to  the  inner  and 
lower  side  of  the  ball,  which  rotates  it  inward 
and  downward. 

Muscular  Mechanism. — These  muscles  act  in 
pairs,  with  and  against  each  other,  and  the  force 
exerted  by  any  pair  is,  in  a  normal  healthy  per- 
son, equally  divided  between  the  two  eyes.  The 
internal  recti  act  together,  pulling  both  eyes  hori- 
zontally inward  with  exactly  equal  effect,  and  are 
opposed  by  the  external  recti,  which  also  act  to- 
gether pulling  the  two  eyes  equally  outward.  The 
superior  oblique  act  together  in  a  similar  fashion, 
and  are  opposed  by  the  inferior  oblique.  It  must 
not  be  supposed  that  these  muscles  are  ordinarily 
at  rest,  only  those  being  in  action  which  are  re- 
quired for  a  given  movement  of  the  eye.  On  the 
contrary,  all  of  the  muscles  are  constantly  in  a 
state  of  contraction ;  when  the  eyeball  is  still,  the 
force  exerted  by  each  pair  is  exactly  equal,  so  that 
no  one  set  overcomes  another;  when  it  is  desired 
to  move  the  eyeball,  a  little  more  nervous  energy 
is  put  into  the  proper  pair  of  muscles,  so  that 
their  action  overcomes  that  of  the  rest  to  just  the 
extent  necessary  to  execute  the  required  movement. 
It  is  important  to  bear  this  in  mind,  as  will  be 
seen  when  we  come  to  study  muscular  insufficiency. 


CHAPTER  IV. 

REFKACTIOISr  OF  THE  EYE. 

The  eye  is  a  refracting  instrument,  with  a  con- 
vex surface,  of  such  density  as  compared  with 
air  that,  in  a  normal  eye  in  a  state  of  rest,  paral- 
lel rays  are  exactly  focused  on  the  retina  at  the 
back  of  the  eyeball. 

This  must  not  be  taken  to  mean,  as  the  ordi- 
narily employed  illustrations  would   seem  to  in- 


lllustrating  how  axial  rays  and  their  divergents 
are  focused  on  the  retina  in  the  normal  eye  at  rest. 
The  divergents  have  become  parallel  with  the  axials 
by  the  time   they  enter   the   eye. 

dicate,  that  rays  of  light  reaching  the  eye  from 
distant  objects  at  infinity  all  fall  upon  its  sur- 
face in  a  horizontal  direction,  so  that  only  one 
horizontal  ray — namely,  that  one  which  travels 
along  the  principal  axis  of  the  eye — is  unrefract- 
ed.  It  has  already  been  shown  how  that  rays 
of  direction  proceed  from  all  points  of  a  distant 
object,  and  pass  through  the  nodal  point  unre- 
fracted.  It  is  the  diverging  rays  accessory  to 
these  ravs  of  direction  which,  by  the  time  they 


48 


REFRACTION 


reach  the  surface  of  the  e3'e,  are  practically  paral- 
lel and  are  refracted  by  the  media  of  the  eye 
so  as  to  meet  their  respective  rays  of  direction 
at  the  retina. 

Index  of  Refraction. — As  a  matter  of  fact 
the  refractive  system  of  the  eye  is  a  compound 


The    normal    or    emmetropic    eye. 

one,  made  up  of  three  separate  media  of  different 
densities  (the  aqueous  humor,  the  vitreous  hu- 
mor, and  the  lens),  and  three  separate  refracting 
surfaces  of  different  convexities  (the  surfaces  of 
the  cornea,  lens,  and  vitreous).  However,  for 
optical  purposes  it  is  convenient  to  regard  the 
eye  as  a  single  refractive  system,  whose  net  re- 
fractive effect  is  the  resultant  of  the  various 
parts.  This  net  index  of  refraction  of  the  eye  is 
about  1.4,  but  naturally  it  varies, 

DiOPTRiSM. — The  refractive  mechanism  of  the 
eye  is  called  its  dioptric  system,  and  the  retina, 


REFRACTION   OF   THE  EYE 


49 


in  a  normal  eye,  is  situated  exactly  at  the  prin- 
cipal focal  point  of  the  dioptric  system. 

Emmetrqpia. — This  condition  is  called  emme- 
tropia;  an  emmetropic  eye  is  one  whose  refrac- 
tion is  normal. 

Ametropia. — When   the   retina   is   not   at   the 


The   hyperopic,    or   short   eye. 

principal  focal  point,  but  is  either  beyond  or  with- 
in it,  so  that  parallel  rays  come  to  a  focus  either 
in  front  of  or  behind  the  retina,  the  eye  is  said 
to  be  Ametropia  An  ametropic  eye  is  one  whose 
refraction  is  abnormal. 

HYPERMETRoriA. — ^Whcu  the  retina  is  situated 
within  the  principal  focal  point,  so  that  parallel 
rays  come  to  a  focus  behind  tlic  retina,  the  eye 
is  said  to  be  Hypermetropic  or  Hyperopic. 

Myopia. — When  the  retina  is  situated  beyond 
'  the   principal   focal   point,   so   that   parallel   rays 


50  REFRACTION 

come  to  focus  in  front  of  the  retina,  the  eye  is 
said  to  be  Myopic. 

Astigmatism. — When  the  refractifig  surface  of 
the  eye  is  irregular,  so  that  all  the  rays  do  not 
focus  at  one  point,  the  eye  is  said  to  be  Astig- 
matic. 


The    myopic,    or   long-   eye. 

Anisometropia. — tWhen  the  eyes  are  both  ame- 
tropic,  but  differently  affected — i.  e.,  when  one 
eye  is  myopic  and  the  other  hypermetropic,  or 
when  one  is  astigmatic  and  the  other  spherically 
abberated,  the  condition  is  called  Anisometropia. 
It  is  highly  important  in  testing  refraction  to 
test  each  eye  separately,  excluding  the  other  from 
vision  meanwhile. 

Errors  of  Refraction. 

The  four  most  common  errors  of  the  eye  which 
the  refractionist  is  called  upon  to  correct  are: 

1.  Hypermetropia  or  Hyperopia. 

2.  Myopia. 

3.  Astigmatism. 

4.  Presbyopia. 


REFRACTION   OF   THE    EYE 


51 


Hypermetropia  is  that  condition  of  the  eye  in 
which  parallel  rays  are  focused  behind  the  retina. 


A 

A. 


^:.---^— P 


Hyponnetropic  or  Short  Eye. — Parallel  rays  of  light, 
AA,  from  distance  focusing-  behind  retina  at  F. 
Dotted  lines — rays  of  light  from  near  object — focus 
still   farther   behind   retina  at   P. 

In  other  words  the  antero-posterior  diameter  of 
the  eyeball,  the  distance  from  the  cornea  to  the 
retina,  is  too  short  in  proportion  to  the  refracting 


Hypermetropic  eye  corrected  by  a  convex  lens, 
which  hastens  refraction  of  the  rays,  and  thus  brings 
the  focal   point   forward.     AA,   rays.     P,   focal   point. 

power  of  the  eye,  and  the  principal  focal  point 
is  back  of  the  retina.  In  order  to  coiTect  this 
error  the  eye  has  to  be  furnished  with  a  spher- 


52 


REFRACTION 


ical  lens  wliicli  will  increase  eye  refraction  and 
hasten  focusing  of  all  the  rays.  In  other  words, 
the  refraction  of  the  eye  must  be  assisted  by  a 


Myoiiif  or  Long-  Kye. — ParaUel  rays  of  lig-lit,  AA, 
focused  too  soon  at  P.  Dotted  lines  show  object 
nearer   the   eye   focused   farther   back. 

lens  of  the  same  curvature  as  the  eye  itself — 
namely,  a  convex  lens.  Convex  lenses  are  there- 
fore called  plus  lenses,  because  they  add  to  the 
refractive  power  of  the  eye. 

Myopia  is  that  condition  of  the  eye  in  which 
parallel  rays  are  focused  in  front  of  the  retina. 
The   antero-posterior   diameter   of   the   eyeball   is 


Showing:  the  myopic  eye  corrected  by  a  concave 
lens,  which  delays  refraction  of  tiie  rays  and  there- 
fore puts   the  focal  point  further  back. 

too  long  in  proportion  to  the  refractive  power 
of  the  eye,  and  the  principal  focal  point  is  in 
front    of   the   retina.      Its   correction   requires    a 


REFRACTION   OF  THE   EYE 


Oo 


spherical  lens  which  will  lessen  eye  refraction 
and  delay  focusing  of  all  the  rays.  That  is  to 
say,  the  refraction  of  the  eye  must  be  antag- 
onized by  a  lens  of  opposite  curvature  to  that  of 
fjiQ  eye — namely,  a  concave  lens.  Concave  lenses 
are  therefore  called  minus  lenses,  because  they 
detract  from  the  refractive  power  of  the  eye. 

Astigmatism  signifies  a  condition  of  the  eye  in 
which  the  curvature  of  the  cornea  is  not  the  same 
in  all  its  meridians,  one  or  two  being  more  or  less 


1 

1 1 '  . i  M  1  i  1 1 1 1 1 TT 

4U--   t=H>nUiltj 

- . ^ — 

ii    

1  [ 

--^P^ 

Tilnstrates    astigmatism,    where    the    rays    entering 
too   soon. 

convex  than  the  rest,  so  that  while  the  rays  from 
the  normal  meridians  are  properly  focused,  those 
from  the  defective  meridians  form  a  line  of  un- 
focused rays,  either  in  front  of  or  behind  the 
retina,  and  a  diffused  indistinct  image  results.  As 
the  name  implies,  it  is  impossil)lc  to  focus  all  the 

rays  at  a  point. 

Chief  Meridians.— The  most  convex  and  tlie 
least  convex  meridians  are  always  precisely  at 
right  angles  to  each  other.  If  one  is  vertical  the 
other  is  horizontal:  if  one  is  at  20  the  other  is 
at  110.    These  are  called  the  chief  meridians,  and 


54 


REFRACTION 


are  tlie  meridians  which  must  be  estimated  and 
corrected. 

The  eye  is  normally  a  little  astigmatic^  the 
vertical  meridian  being  naturally  a  trifle  more 
convex  than  the  horizontal,  but  as  long  as  the 
difference  is  not  sufficient  to  interfere  wth  clear 
vision  it  is  regarded  as  normal  and  not  corrected. 

In   pathological   astigmatism   the   relative  con- 


Illustrates  the  correction  of  the  foregoing,  by  a 
concave  cylinder  with  its  axis  at  right  angles  to  the 
defective  meridian.  The  raj'^s  entering  the  cylinder  at 
right  angles  to  its  axis  are  refracted  and  their  focal 
point  carried  back  to  coincide  with  that  of  the  normal 
meridian. 


vexity  of  the  meridians  is  usually  the  same  as 
in  normal^ — i.  e.,  the  vertical  and  horizontal 
meridians  are  usually  the  two  chief  meridians, 
and  the  vertical  is  usually  the  more  convex,  and 
the  astigmatism  is  then  said  to  be  "with  the 
rule."  But  of  course  there  are  frequent  excep- 
tions to  this  rule,,  in  which  case  the  astigmatism 
is  said  to  be  "against  the  rule." 

Presbyopia  is  really  a  phase  of  hypermetropia, 
but  the  name  is  used  to  indicate  that  form  of 
hypermetropia  which  depends  upon  the  effects  of 
age  upon  the  crystalline  lens,  hardening  it  and 


REFRACTION  OF  THE  EYE  55 

thus  preventing  the  ciliary  muscle  from  perform- 
ing its  accommodating  function.  Presbyopia  is 
arbitrarily  said  to  begin  when  the  patient^s  near 
point  has  receded  to  22  cm.  This  usually  occurs 
at  about  45  years  of  age^  and  increases  about  1  D. 
for  every  subsequent  five  years.  Its  correction  is, 
of  course,  accomplished  by  convex  lenses. 


CHAPTER  V. 

LENSES. 

Lenses  rire  nowadays  usually  cut  out  of  crown 
glass,  pebbles  being  very  expensive,  hard  to  grind, 
and  possessing  no  particular  advantages  over 
glass.  They  are  of  twojdnds  as  to  curvature — 
convex  and  concave. 


Convex  Snrfnee 


Shows  how  rays  entering-  a  convex  lens,  and  being- 
bent  toward  the  perpendicular,  converge.  Note  that 
the  focal  point  is  where  the  refracted  rays  actually 
meet    (positive). 

In  pursuance  of  the  principle  that  rays  of  light 
entering  a  denser  jnediujn.  are  bent  toward  the 
perpendicular  of  the  refracting_surface^  rays  which 
enter  a  convex  lens,  other  than  the  ray  which 
traverses  the  principal  axis^  are  bent  toward  each 
other — i.  e.,  so  as  to  converge — while  those  which 
enter  a  concave  lens,  other  than  along  the  prin- 
cipal axis,  are  bent  away  from  eacli  other — i.  e., 
so  as  to  diverge. 

The  focal  point  oJ__aJlens  is  the  point  at  which 
parallel  rays,  refracted  by  the  lens,  are  brought 
to  a  focus. 


58  BEFEACTION 

The  focal  point  of  a  convex  lens  is  the  point 
at  which  the  refracted  rays  actually  come  to  a 
focus^  and  is  called  positive. 

The  focal  point  of  a  concave  lens  is  the  point 
at  which  the  refracted  rays  would  meet  if  pro- 
jected backward  on  the  concave  side  of  the  lens, 
and  is  called  negative.  This  is,  of  course,  pre- 
cisely the  opposite  to  the  focal  point  of  a  con- 
vex or  concave  mirror  for  reflected  rays. 


Focal    Point 

(Kegafive) 


Shows  how  rays  entering  a  concave  lens,  and  being 
bent  toward  the  perpendicular,  diverge.  Note  that 
the  focal  point  is  where  the  refracted  rays  would 
meet  if  projected  backward  on  the  concave  side  of  the 
lens    (negative). 


The  focal  length  of  a  lens  is  the  distance  be- 
tween the  refracting  surface  and  the  focal  point. 

Lenses  are  also  spherical — i.  e.,  segments  of  a 
sphere — and  cylindrical — i.  e.,  segments  of  a  cyl- 
inder. There  are  convex  and  concave  spherical  s 
and  convex  and  concave  cylindricals. 

Spherical  lenses  refract  all  rays  except  one 
(the  principal  axis),  and  therefore  converge  them 
all  toward  or  diverge  them  all  away  from  a  point, 
the  resulting  path  of  the  rays  being  cone-shaped. 


LENSES 


59 


Cylindrical  lenses  refract  only  those  rays  which 
strike  them  at  right  angles  to  their  axis;  those 
which  strike  them  in  the  same  line  as  their  axis 
are  perpendicular  to  the  refracting  surface  and 
therefore  pass  through  without  any  change  of  di- 
rection.    Cylindrical  lenses  therefore  converge  or 


Showing  how  rays  which  enter  a  cylinder  paraHel 
to  its  axis  are  unchanged,  while  those  which  enter  at 
right  angles  to  the  axis  are  refracted. 


diverge  a  straight  line  of  rays  toward  or  away 
from'  a  point,  the  resulting  path  of  the  refracted 
rays  being  fan-shaped. 

DioPTRiSM. — The  refracting  power  of  a  lens  is 
called  its  dioptrism.  Inasmuch  as  the  density  of 
the  glass  is  the  same  in  all  lenses,  this  dioptrism 
is  dependent  upon  the  curvature  of  their  surfaces 
— the  greater  the  convexity  or  concavity,  the 
greater  their  dioptrism. 

A  lens  having  a  focal  length  of  1  meter — i.  e., 
which  brings  parallel  rays  to  a  focus  at  a  dis- 
tance of  1  meter  from  its  surface — is  taken  as  a 


GO 


REFRACTION 


standard^  and  its  strength  designated  as  1  diop- 
ter, or  1  D. 

The  dioptrism  increases  in  inverse  geometrical 
ratio  to  the  focal  length.  Manifestly  a  lens  whose 
focal  length  is  half  a  meter  has  twice  the  curva- 
ture, and  therefore  twice  the  dioptrism,  of  one 
whose  focal  len.gth  is  1  meter.  Therefore  a  lens 
whose  focal  length  is  half  a  meter  is  2  D,  a  quar- 
ter of  a  meter  4  D,  etc. 

The  diopter  of  every  lens  and  its  focal  length 
are  given  in  the  trial  case.   The  lenses  are  graded, 


'2545  6 

Shows  different  methods  of  grinding-  spherical  lenses. 

as    a    rule,    in    fractions    of    0.25    of  a  diopter, 
which  are  quite  convenient  to  work  with. 

Gkindinct    of    Lenses. — Spherical    lenses    are 
ground  in  the  following  manner: 
'  1.     Plano-convex. 

2.  Bi-convex. 

3.  Converging  concavo-convex. 

4.  Plano-concave. 

5.  Bi-concave. 

6.  Diverging  concavo-convex. 
CoNCAVQ-CoNVEx  Lenses. — It  will,  of  course, 


LENSES  61 

be  seen  that  in  the  concavo-convex  form  of  lens 
(by  far  the  most  commonly  used,  by  the  way,  of 
all  the  different  forms)  the  net  effect  of  the  lens 
depends  upon  the  relative  curvature  of  the  enter- 
ing and  merging  surface.  If  both  entering  and 
emergin.o-  surfaces  were  equally  curved,  the  ray 
would  be  refracted  upon  entering,  and  exactly 
neutralized  again  when  it  emerged,  only  being 
displaced  laterally.  If  the  entering  surface  is 
more  curved  than  the  emerging,  the  net  result 
is  that  of  a  convex  lens;  if  the  emerging  surface 
has  the  greater  curvature,  the  net  effect  is  that 
of  concavity. 

Cylindrical  leniiLs  arc  always_  gi'ound  with  the 
reverse  side  piano,  except  when  they  are  com- 
bined with  spherical  lenses  for  the  correction  of 
compound  and  mixed  astigmatism,  in  which  case 
the  spherical  correction  is  ground  on  the  reverse 
side. 


CHAPTER  VI. 

ACCOMMODATION    AND    CONVERGENCE. 

The  Eye  at  Rest. — When  the  noi-mal  eye  is  at 
rest,  parallel  rays— i.  e.,  rays  that  originate  six 
meters  or  more  from  the  observer — are  focused 
on  the  retina;  hence  in  this  condition  objects  at 
this  distance  are,  so  far  as  refraction  is  con- 
cerned, clearly  seen. 


inustrates  how   divergent   (finite)    rays   entering  the 
normal   eye  at   rest   are   focused   back   of   tlie   retina. 

It  is  evident  that  under  the  same  condition  of 
refraction  divergent  rays — i.  e.,  rays  that  pro- 
ceed from  a  less  distance  than  six  meters — do 
not  focus  on  the  normal  retina,  but  are  carried 
beyond  it  and  come  to  a  focus  behind  it.  Hence 
objects  at  a  distance  of  less  than  six  meters  are 
not  clearly  seen  by  the  normal  eye  in  a  state  of 

rest. 

Accommodation.— In  order  that  the  eye  may 
clearly  see"  objects  at  a  finite  distance  the  refrac- 
tive power  of  the  eye  must  be  increased.  This 
is  accomplished  by  the  contraction  of  the  ciliary 


64  REFRACTION 

muscle,  drawing  forward  the  choroid  and  releas- 
ing the  suspensor}^  ligament  of  the  lens,  whose 
elasticity  then  changes  its  shape  so  as  to  increase 
the  convexity  of  the  refracting  surface.  This 
power  of  changing  the  focus  of  the  eye  is  called 
accommodation. 

Far   Point. — Tlie  point  from  which   rays  of 
light  will  focust  upon  the  retina  of  an  eye  whose 


lUustrates  how  the  eye  accommodates  itself  to 
focus  divergent  rays  on  the  retina.  Note  the  increased 
convexity  of  the  lens  (produced  by  contraction  of  the 
ciliary  muscle). 

ciliary  muscle  is  relaxed  to  its  fullest  possible-  ex- 
tent is  called  its  far  point,  or  punctum  remotum. 

Inasmuch  as  the  normal  eye,  with  its  ciliary 
completely  relaxed,  is  adapted  for  focusing  in- 
finite— i.  e.,  parallel — rays,  it  follows  that  the  far 
point  of  a  normal  eye  is  at  infinity — i.  e.,  at  six 
meters  and  beyond. 

It  must  be  remembered  that  in  the  matter  of 
accommodation  any  point  beyond  six  meters  is 
equally  at  infinity.  The  normal  relaxed  eye  is, 
so  far  as  its  accommodation  is  concerned,  adapted 
for  an  image  of  an  object  six  miles  off  equally  as 


ACCOMMODATION    AND    COVERGENCE  6o 

well  as  for  that  of  an  object  six  meters  distant. 
Wliat  difference  there  is  in  the  vision  of  two 
such  objects  is  due  to  other  factors,  such  as  in- 
tensity, visual  angle,  etc. 

Near  Point. — The  point  from  which  rays  of 
light  will  focus  upon  the  retina  of  an  eye  whose 
ciliary  muscle  is  contracted  to'  its  fullest  possible 
degree  is  called  its  near  point,  or  punctum  proxi- 
mum. 

The  near  point  is  usually  ascertained  by  meas- 
uring- the  least  distance  at  which  the  subject  can 
read,  with  each,  eye  _separately,  the  reading  type 
No.  1  whiclTiFconstructed  according  to  the  visual 
angle. 

Or  it  may  be  found  by  placing  in  front  of  the 
eye  a  piece  of  card  pierced  with  two  small  holes, 
not  farther  apart  than  the  diameter  of  the  pupil 
(the  other  eye  being  excluded  meantime),  and 
directing  the  patient  to  look  at  a  small  object, 
say  a  pin  head,  which  is  gradually  moved  nearer 
to  the  eye. 

As  long  as  the  accommodation  is  able  to  focus 
the  rays  from  the  pin  which  pass  through  the 
holes  in  the  card,  they  will  form  a  single  image 
on  the  retina;  but  as  soon  as  the  near  point  is 
passed,  so  that  the  rays  become  too  divergent  for 
accommodation  to  focus,  they  will  form  two  sep- 
arate images  and  the  eye  will  see  two  pins. 

Range  of  Accommodation. — The  distance  be- 
tween the  far  and  near  points  of  an  eye  is  called 
its  range  of  accommodation. 


66  REFRACTION  * 

Amplitude  of  Accommodation. — The  mus- 
cular and  nervous  energy  necessary  to  change  the 
eye  from  its  far  to  its  near  point  i.s  called  its 
amplitude  of  accommodation. 

Lens  Measurement  of  Accommodation. — It 
has  already  been  seen  that  the  act  of  accommoda- 
tion consists  in  a  muscular  increase  in  the  con- 


NEAR  POINT 


Illustrating-  how  a  convex  lens  which  renders  x'ays 
from  the  near  point  equivalent  to  those  from  the  far 
point,  focusing-  thein  on  the  retina  is  the  measure  of 
the   accommodation    of   the   eye. 

vexity  of  the  crystalline  lens,  by  which  refraction 
of  the  finite  (divergent)  rays  is  increased  and  fo- 
cusing hastened. 

If  the  refraction  of  these  rays  be  assisted  by 
a  convex  glass  lens  held  before  the  eye,  the  ac- 
commodative effort  of  the  eye  will  be  spared  to 
that  extent;  and  it  is  possible  to  find  a  convex 
glass  lens  which  will  spare  the  eye  its  accommo- 
dative effort  altogether,  so  that  by  the  help  of 
such  a  lens  rays  from  its  near  point  will  focus 
on  the  retina  without  any  accommodation  at  all, 
just  as  though  they  came  from  the  far  point.  It 
\\ill  readily  be  seen  that  the  dioptric  strength  of 


ACCOMMODATION    AND    COVERGENCE  67 

the  convex  lens  which  accomplishes  this  is  the 
exact  measure  of  the  entire  amplitude  of  accom- 
modation of  the  eye. 

Conversed,  the  distance  from  the  eye  to  its 
near  point  is  the  focal  length  of  the  convex  lens 
whose  dioptric  strength  corresponds  with  the 
amplitude  of  accommodation. 

XoRMAL  Amplitude  of  Accommodation. — 
Ordinarily  the  range  and  amplitude  of  accommo- 
dation are  about  the  same  for  all  healthy  eyes  at 
a  given  age.  The  following  table  shows  the  ampli- 
tude at  various  ages: 

10  years 14      D. 

15  years 12      D. 

20  years 10      D. 

30  years 7      D. 

40  years 4.5   D. 

50  yeai^ 2.5  D. 

60  years 1      D. 

75  years 0 

The  reason  for  the  decrease  of  amplitude  as 
age  advances  is  that  the  capsule  of  the  crystal- 
line lens  loses  its  elasticity,  so  that,  although  the 
ciliary  contracts  and  releases  the  lens  from  the 
suspensory  ligament^  it  is  no  longer  able  to  as- 
sume so  convex  a  form.  It  is  probable,  too,  that 
with  advancing  age  the  ciliary  muscle  itself  be- 
comes thus  capable  of  contraction. 

Absolute  and  Binocular. — The  amount  of 
accommodation  which  one  eye  can  exert  when  the 
other  is  shut  out  of  vision  is  called  absolute  ac- 
commodation.    That  which  both  eyes  together  can 


68 


REFRACTION 


exert  is  called  hinocular  accommodation.  The  lat- 
ter is  a  little  more  than  the  former,  so  that  in 
testing  for  glasses,  after  testing  each  e3^e  sep- 
arately it  will  usually  be  found  that  both  eyes  to- 
gether can  take  a  little  stronger  convex  and  a  lit- 
tle weaker  concave  correction  than  each  eye  sepa- 
ratelv  calls  for.   - 


YpIIow   Spot 


Yellow   Spot 


Rectus  Muscle 


Illustrates  how  the  eyes  at  rest  have  their  yellow 
spots  adjusted  for  parallel  rays  and  need  no  converg- 
ence. 

Convergence. 

Convergence  is  a  necessary  element  in  near 
binocular  vision,  and  is,  therefore,  intimately  as- 
sociated with  accommodation.  It  is,  however, 
quite  independent  of  it,  and  paralysis  of  one  does 
not  affect  the  other. 

Convergence  is  the  power  of  directing  the  visual 
axes  of  the  two  eyes  toward  a  point  nearer  than 
infinity.  In  order  to  get  singleness  of  binocular 
vision  the  image  formed  on  the  central  parts  of 


ACCOMMODATION     AND     COVERGENCE  69 

the  retina  must  exactly  coincide  in  the  two  eyes, 
or  there  will  be  double  vision.  For  objects  at 
infinity  the  visual  axes  of  the  eyes  at  rest  are 
adjusted  to  accomplish  this;  but  for  objects  with- 
in a  finite  distance  it  is  necessary  to  direct  the 
yellow  spot  in  each  eye  toward  the  same  point. 
This  is  done  by  converging — i.  e.,  pulling  the  eyes 
inward  by  means  of  the  internal  recti  muscles. 


Point 

Illustrates  the  pulling  of  the  «y^s  inward  (converg- 
ence) by  the  internal  recti  to  direct  the  yellow  spots 
toward  the  same  near  point. 

Range  of  Convergence  is  the  extent  of  adduc- 
tion and  abduction  capable  of  being  accomplished 
by  the  internal  and  external  recti  respectively. 
The  former  is  known  as  positive  convergence,  the 
latter  as  negative  convergence. 

Amplitude  of  Convergence  is  the  amount  of 
muscular  and  nervous  energy  necessary  to  change 
the  eye  from  extreme  adduction  to  extreme  ab- 
duction, or  the  reverse,  the  former  being  known 
as  amplitude  of  positive  convergence,  the  latter 
amplitude  of  negative  convergence. 


70  REFRACTION 

Prism  Tests. — The  amplitude  of  convergence 
is  measured  by  means  of  prisms,  of  which  every 
trial  case  contains  a  sufficient  supply. 

According  to  the  laws  of  refraction  already 
enunciated,  prisms  deflect  rays  of  light  toward 
their  base,  both  upon  their  entering  and  leaving 


Illustrates  how  a  ray  is  bent  toward  the  base  both 
on   entering-  and  on  emerging  from  a  prism. 

the  medium.  Upon  striking  the  surface  of  the 
prism,  i.  e.,  upon  entering  a  denser  medium,  the 
ray  is  bent  toward  the  perpendicular,  which  is 
toward  the  base.  Upon  passing  out  of  the  prisms 
into  the  air  again,  i.  e.,  upon  entering  a  rarer 
medium,  it  is  bent  away  from  the  perpendicular, 
and  that  is  also  toward  the  base. 

The  optical  effect  of  a  prism  is,  of  course,  to 
displace  the  image  away  from  the  base  and  to- 
ward the  apex  of  the  prism,  the  object  appearing 
to  be  in  a  straight  line  from  the  eye  through  the 
point  of  convergence,  and  as  far  from  the  e3^e  as 
the  light  has  actually  traveled. 

It  will  readily  appear  that  a  prism  placed  be- 
fore the  eve  with  its  base  outward  will  focus  the 


ACCOMMODATION     AND     COVERGENCE 


71 


central  rays  on  the  outer  side  of  the  yellow  spot 
of  that  eye,  while  in  the  uncovered  eye  they  will 
fall  upon  the  yellow  spot,  thus  producing  double 
vi^^ion.      To    overcome    this,    the   internal    rectus 


Illustrates  how  a  prism,   base  out,   focuses  a  ray  on 
the  outer  side  of  the  yellow  spot. 


must  contract  and  pull  the  eye  inward  until  the 
images  on  both  retinae  coincide  as  to  position. 
This  compensation^  however,  is  not  accomplished 
solely  by  one  eye,  inasmuch  as  one  rectus  cannot 
act  independently  of  its  mate.  Therefore,  in- 
stead of  the  rectus  of  the  prismed  eye  pulling  that 


72- 


REFRACTION 


eye  inward  until  the  deflected  rays  focus  on  its 
yellow  spot,  both  eyes  are  pulled  inward  until 
the  image  falls  upon  corresponding  retinal  points 
midway  between  the  yellow  spot  and  that  of 
prismatic  deflection^  thus  dividing  the  effect  of 
prism  equally  between  the  two  muscles.  A  prism 
of  given  strength  held  before  one  eye  is  equal  in 
effect  to  two  prisms  of  half  that  strength  held 
before  both  eyes. 


Illustrating-  the  metric  angle  made  l)y  various  visual 
axes,  M'E  M"E  M"'E,  with  the  central  line  MH.  Note 
that  the  parallel  lines  AE.  A'E'  (eye  at  rest)  make 
no  ang-le  at  all,  while  BE,  B'E'  (eyes  divergent)  make 
negative  angles.  ECH  and  E'CH  which  are  measured 
by  prolonging  the  central  line  and  axes  to  meet  be- 
hind the   eyes  at  C. 


The  same  conditions  apply  to  the  external  mus- 
cles under  the  influence  of  prisms  with  their  base 
inward. 

Measure  of  Convergence. — Prisms  are  num- 
bered according  to  the  degree  of  their  angle,  and 
the  angle  of  the  prism  which  can  be  overcome  by 
the  recti  muscles,  i.  e.,  the  strongest  prism  with 
which  single  vision  can  be  miantained  is  the 
measure  of  the  amplitude  of  convergence. 

The  amount  of  deviation,  measured  in  angles, 


ACCOMMODATION    AND    COVERGENCE  73 

produced  in  both  eyes  by  a  given  prism  or  pair  of 
prisms,  is  equal  to  half  the  total  prismatic  angle; 
and  as  the  deviation  is  equally  divided,  between 
the  two  eyes  the  deviation  of  each  eye  is  equal  to 
one-fourth  of  the  total  prismatic  angle.  Thus,  a 
prism  of  8°,  or  two  prisms  of  4°,  will  produce  a 
total  deviation  of  4°,  or  a  deviation  of  2°  in 
each  eye. 

For  example,  a  prism  of  8°,  base  out,  before  the 
right  eye,  will  cause  the  central  rays  to  focus  4° 
to  the  outer  side  of  the  yellow  spot  in  that  eye. 
In  order  to  overcome  the  effect  of  this  prism,  and 
obtain  single  vision,  the  internal  recti  muscles 
will  pull  each  eye  inward  2°,  so  that  the  rays  will 
focus  on  each  retina  2°  outside  the  yellow  spot. 

Metric  Angle. — The  degree  of  convergence  is 
sometimes  expressed  in  meters,  the  metric  angle 
being  the  angle  formed  by  the  visual  axis  witli 
the  median  line,  i.  e.,  an  imaginary  line  drawn 
perpendicular  to  the  base-line  joining  the  points 
of  rotations  of  the  two  eyes  midway  between 
them.  A  metre  is  taken  as  the  unit  of  this  me- 
dian line,  the  metric  angle  of  convergence  for  a 
point  one  metre  away  being  called  1,  half  a  metre 
away  2,  two  metres  away  i/^,  and  so  on.  Wlien 
the  object  is  at  infinity  the  visual  axis  and  the 
median  line  are  of  course  parallel  and  the  metric 
angle  is  then  infinity  or  zero. 

Far  Point. — ^The  distance  for  which  the  con- 
vergence is  adapted  when  the  angle  of  converg- 
ence is  at  its  minimum  is  called  the  fai'  point,  or 


74  REFRACTION 

punctum  remotum,  of  convergence.  Theoretically 
this  point  is  infinity,  but  actually  the  visual  axes 
of  most  eyes  in  this  state  of  rest  are  divergent, 
so  that  the  metric  angle  is  a  negative  one.  In 
this  case  the  distance  of  the  far  point  is  found  by 
projecting  the  lines  of  the  visual  axes  backwards 
until  they  meet  behind  the  eye. 

Near  Point. — When  the  metric  angle  is  at  its 
maximum,  i.  e.,  when  the  eyes  are  converged  to 
their  utmost  capacity,  the  distance  for  which  they 
are  then  adapted  is  called  the  near  point,  or  punc- 
tum proximum,  of  convergence. 

Amplitude  of  positive  convergence  is  meas- 
ured by  the  strongest  prism,  base  out,  with  which 
single  vision  can  be  maintained.  This  indicates 
the  greatest  amount  of  deviation  of  which  the 
internal  recti  muscles  are  capable.  A  normal 
pair  of  eyes  should  be  able  to  overcome  one  prism 
of  20  to  30  deg.  or  two  prisms  of  10  to  15  deg. 
each. 

Amplitude  of  negative  convergence  is  meas- 
ured by  the  strongest  prism,  base  in,  with  which 
single  vision  can  be  maintained.  This  indicates 
the  greatest  amount  of  deviation  which  the  exter- 
nal recti  are  capable  of  producing.  Normally  the 
eyes  can  overcome  one  prism  of  6  to  8  deg.  or 
two  of  3  to  4  deg.  each. 

Ahsolute  negative  convergence,  i.  e.,  in  which 
the  metric  angle  is  negative,  is  measured  by  the 
strongest  prism,  base  in,  with  which  single  vision 
can  be  maintained  of  an  object  at  six  meters. 


acco:mmodation  and  covergence  75 

Insufficiency  of  Convergence. — Anything 
short  of  the  ahove-named  normal  degrees  of  ampli- 
tude indicates  weakness  or  imbalance  of  the  ex- 
trinsic ocular  muscles^  which  must  be  further  in- 
vestigated and  dealt  with  as  laid  down  in  the 
chapter  on  that  subject. 

Ratio  of  Convergence  and  Accommodation. 
Convergence  and  accommodation  normally  in- 
crease and  decrease  in  mutual  ratio,  since  the 
nearer  an  object  is,  the  more  one  has  to  converge 
to  see  it,  and  the  more  accommodation  one  has  to 

use. 

Indeed,  the  simple  metric  and  dioptric  system 
already   explained    of   expressing   degrees   of   ac- 
commodation and  convergence  enables  us  to  ob- 
serve a  very  uniform  ratio  between  the  two  func- 
tions.   Thus  when  vision  is  adapted  for  an  object 
at  1  meter  distance  the  convergence,  as  expressed 
by  the  metric  angle,  is  1,  and  the  accommodation, 
as  expressed  in  diopters,  is  1  D.   For  an  object  at 
infinity  convergence  and  accommodation  are  both 
infinity.     For  an  object  at  2  meters  convergence 
is   1/2   and   accommodation   0.50   D.      At  half   a 
meter  convergence  is  2  and  accommodation  2  D. 
The  amplitude  of  convergence  is,  however,  nor- 
mally a  little  greater  than  that  of  accommodation, 
both  near  and  far.     Below  is  a  table  showing  the 
metric  angle  for  various  distances  and  the  actual 
values  of  the  metric  angle  in  degrees  of  the  circle. 
The  table  is  based  on  an  average  distance  between 
the  centers  of  rotation  of  the  two  eyes  of  6.4  cm. 


76 

REFBACTIO:^ 

Distance 

of  the  Object 

The  Metrical 

Value  Expressed 

from 

the  Eyes. 

Angle. 

in  Degrees. 

1 

metre 

1 

1° 

50' 

50 

cm 

2 

3° 

40' 

33 

cm 

3 

5° 

30' 

25 

cm 

4 

7° 

20' 

20 

cnn 

5 

9° 

10' 

16 

cm 

6 

11° 

14 

cm 

7 

12° 

50' 

12 

cm 

8 

14° 

40' 

11 

cm 

9 

16° 

30' 

10 

cm 

10 

18° 

20' 

9 

cm 

11 

20° 

10' 

8 

cm 

12 

22° 

7.5  cm 

13 

23° 

50' 

7 

cm 

14 

25° 

40' 

6.5   cm 

15 

27° 

30' 

6 

cm 

16 

29° 

20' 

5.5   cm 

18 

33° 

5 

cm 

20 

36° 

40' 

It  is  readily  seen  that  in  cases  of  hyperme- 
tropia  and  myopia  the  direct  ratio  between  ac- 
commodation and  convergence  is  disturbed,  the 
hypermetrope  using  less  convergence  and  more  ac- 
commodation, and  the  myope  more  convergence 
and  less  accommodation.  This  will  be  more  clear- 
h^  apparent  when  we  come  to  deal  with  these  er- 
rors of  refraction. 


CHAPTER  VII. 

EETINOSCOPY. 

Retinoscopy,  skiascopy,  or  the  shadow  test,  has 
long  been   a  popiilnr   method  of  estimating  and 


The  lieiiiioscope. 


correcting  refraction  among  European  ophthal- 
mologists, and  is  rapidly  gaining  favor  in  this 
coimtrv.      Xo   refractionist   should   allow   a   case 


78  REFRACTION 

to  pass  out  of  his  hands  without  subjecting  it 
to  this  valuable  test.  It  has  the  distinct  advan- 
tages of  being  entirely  objective  in  character,  i.  e., 
not  at  all  dependent  upon  what  the  patient  may 
or  may  not  see  or  think  he  sees,  simple  to  carry 
out,  and  accurate  in  results. 

The  retiiioscope  itself  is  a  very  plain  instru- 
ment, consisting  simply  of  a  round  mirror  about 


ONE    METER 


T^ight    over    patient's    head    and    the    observer    with 
mirror   at   one-meter  distance. 


3  cm.  in  diameter,  mounted  on  a  stem  handle,  and 
having  a  sight  hole  through  the  center  of  the  mir- 
ror. There  are  several  different  makes  of  retino- 
scopes  on  the  market,  any  one  of  which  is  serv- 
iceable; but  for  reasons  which  are  elsewhere  ex- 
plained, it  is  best  to  use  a  concave  mirror  of  25 
cm.  focal  length. 

The  principle  of  the  retinoscope  is  that  it  per- 
mits the  rays  emerging  from  the  pupil  of  the 
subject's  eye  to  be  intercepted  in  their  path  by 
the  observers  eye,  a  thing  which,  as  already  ex- 


RETINOSCOPY 


79 


plained,  cannot  be  done  without  some  such  con- 
trivance. The  light  thrown  into  the  subject's 
pupil  from  the  mirror  of  the  retinoscope  natural- 
ly returns  again  by  the  same  path,  and  is  inter- 
cepted by  the  mirror.  But  the  sight  hole  enables 
the  observer  to  place  his  own  eye  right  in  this 


Illustrates  the  kind  of  lamp  for  retinoscopy. 

path  of  the   returned   rays,   and  thus  to   receive 
them  upon  his  own  retina. 

The  examination  should  take  place  in  a  dark 
room,  especially  until  the  operator  is  practiced 
and  experienced,  so  that  the  illumination  of  the 


80 


REFRACTION 


patient's  pupil  may  stand  out  in  bold  relief  from 
the  surrounding  darkness,  and  the  observation 
thus  be  rendered  easier. 

The  best  possible  light,   in  the  writer's  judg- 
ment, is  an  electric  bulb  of  at  least  32  C.  P.,  and 


Illustrates    the    technic    of    retinoscopy. 

cylindrical  chimney,  but  gas  or  an  oil  frame  will 
do  very  well.  TOiichever  light  is  used,  however, 
it  should  have  a  cylindrical  opaque  cover  with  a 
circular  bull's-eye  aperture,  and  over  this  aper- 
ture a  diaphragm  with  a  circular  hole  10  mm.  in 
diameter  for  the  light  to  emerge. 

The  Z?(yr/ir  should  be  placed  five  or  six  inches 


UETINOSCOPY  81 

in  front  and  to  the  left  side  of  the  observer,  fac- 
ing so  that  its  rays  Avill  fall  upon  the  mirror  when 
held  before  the  observer's  right  eye,  but  will  leave 
his  other  eye  in  the  shadow.  The  nearer  the 
source  of  light  is  to  the  mirror,  consistent  with 
the  conditions  already  described,  the  better,  as  the 
illumination  will  then  be  the  more  brilliant. 

The  observer  seats  himself  or  stands  about  one 
meter  from  the  patient.  The  source  of  light,  tlie 
observer's  eye,  and  the  patient's  eye  should,  if 
possible,  be  all  in  the  same  horizontal  plane; 
the  more  nearly  this  rule  is  observed  the  simpler 
will  be  the  operation  and  the  more  reliable  the 
observation. 

The  observer  now  applies  his  right  eye  to  the 
sight  hole  (at  the  back  of  the  mirror,  of  course) 
and  throws  the  reflected  light  from  the  lamp  into 
the  patient's  pupil.  This  is  at  first  quite  a  diffi- 
cult feat,  but  can  be  accomplished  by  first  finding 
the  bright  disc  of  reflected  light,  and  then  slowly 
and  carefully  bringing  it,  by  appropriate  manipu- 
lation of  the  mirror,  onto  the  patient's  face,  and 
finally  onto  his  pupil.  The  modern  electric  retin- 
oscope,  fitted  with  its  own  electric  light,  does  away 
with  this  difficulty. 

The  observer's  eye  being  directly  in  the  path 
of  the  rays  as  they  are  reflected  back  from  the 
patient's  pupil,  the  fundus  of  the  patient's  eye 
is  seen  as  a  red  glare;  this  is  called  the  fundus 
reflex. 


S'2  ){i:m{A(  I'loN 

The  Shadow. — Xow  tlic  operator  rotates  his 
head,  toorether  witli  tlio  jiiirror,  very  sliojhtlv  to 
and  fro^  from  one  side  to  tlie  other,  meantime 
carefully  watehiiig  Ihe  patient's  pupil,  and  a 
shadow  will  be  seen  to  appear  alternately  from 
each  side  of  [lie  pupil  and  iiiove  across  the  illu- 
minated area.  This  shadow  moves  either  in  the 
same  direction  as  the  mirror  is  rotated,  or  in  the 
opposite  direction.  In  technical  language,  it 
moves  either  with  or  against  the  mirror. 

As  a  matter  of  fact,  of  course,  it  is  not  the 
shadow  that  moves  at  all,  but  the  illuminated 
image  of  the  light  thrown  by  the  mirror,  but  as 
the  shadow  follows  the  image  and  is  more  readily 
seen  than  the  light-image,  we  give  our  attention 
to  the  movements  of  the  shadow. 

Eeasox  of  ^Movements. — The  movement  of 
the  shadow  with  or  against  the  mirror  depends 
upon  whether  the  rays  of  light  that  emerge  from 
the  patient's  pupil  have  met  and  crossed  before 
they  reach  the  observer's  eye,  or  not.  If  they 
have  not  yet  crossed,  it  is  manifest  that  the  image 
seen  by  the  observer  will  be  a  "virtual  erect'^ 
image  which  will  move  in  just  tlie  opposite  direc- 
tion from  the  luirror — on  })reeisely  the  same  prin- 
ciple that  the  image  is  seen  to  move  through 
a  convex  lens,  as  explained  in  a  former  chapter. 

If,  on  the  other  hand,  the  rays  have  crossed  by 
the  time  they  reach  the  observer's  eye,  the  image 
will  be  an  inverted  and  real  one,  formed  between 


UETINOSCOPY 


83 


tlie  patient's  eye  and  the  mirror^  which  will  move 
in  the  same  direction  a^  the  mirror. 


ONE    METER 


Illustrates  how  parallel  emerging  rays  from  an  em- 
metropic eye  never  meet,  so  that  when  they  reach 
the  retinoscope  they  are  still  uncrossed,  causing  the 
shadow    to    move   against   the   mirror. 

Point  of  Eeversal. — The  point  in  front  of 
the  patient's  eye  where  the  rays  emerging  from 
the  pupil  meet  and  cross,  and  where  the  image 


Showing  how  parallel  emerging  rays  from  an  emme- 
tropic eye  are  brought  to  a  focus  at  1  meter  by  a 
convex  lens  of  1  D.  bringing  the  point  of  reversal 
exactly   at    the   retinoscope. 

therefore  changes  from  a  virtual  erect  image  to 
an  inverted  real  (mo,  as  evidenced  hy  the  change 


84  REFRACTION 

in  its  moA'cments  in  relation  to  those  of  the  mir- 
ror, is  the  point  for  which  we  seek  in  rctinoseopy. 
Or,  to  speak  more  correctly,  we  seek  to  bring  the 
emerging  rays  to  a  crossing  point  at  a  certain 
lixed  distance  from  the  patient's  eye,  and  by  the 


h)i. owing-  how  divergent  emerging  rays  fi.  ni  a  li5'per- 
opic  eye  are  still  uncrossed  when  they  reach  the 
retinoscope,  causing  the  shadows  to  move  against  the 
mirror. 


nature  and  strength  of  tlie  lens  which  we  are 
obliged  to  use  to  do  this  we  judge  of  the  refrac- 
tion of  the  patient's  eye. 

Emmetropic  Point  of  Eeversal. — It  is  man- 
ifest that  rays  of  light  emerging  from  a  normal 
emmetropic  eye  in  a  s1?ate  of  rest  are  parallel,  and 
will  therefore  never  meet  and  cross.  It  is  also 
plain,  from  a  consideration  of  what  has  already 
been  said  about  dioptrism^  that  a  convex  lens  of 
1  D  placed  immediately  before  the  patient's  eye, 
will  bring  these  parallel  rays  to  a  focus  just  1 
meter  in  front  of  the  eye. 

Therefore,  if  the  observer's  eye  be  just  one 
meter    (or  actually   jnst   a   trifle  more  than  one 


RETINOSCOPY 


85 


meter,  to  allow  for  the  distance  between  the  pa- 
tient's retina  and  the  lens),  from  the  patient's 
eye,  and  no  lens  be  held  before  the  latter,  the 
rays  will  still  bo  ])arallel  when  they  reach  the 
observer,  and  ilie  shadow  will  move  against  the 
mirror. 


Showing-  how  divergent  emerging-  rays  from  a  hyper- 
opia eye  are  brouglit  to  a  focus  at  1  meter  by  a  con- 
vex lens  as  mucli  stronger  than  1  D.  as  the  rays  are 
divergent. 


But  if  a  convex  lens  of  1  D  be  mounted  before 
the  patient's  eye,  tlie  rays  will  be  brought  to 
focus  and  cross  at  1  meter^  i.  e.,  where  the  ob- 
server's eye  is  situated,  so  that  if  he  move  just  a 
trifle  nearer  the  shadow  will  move  against,  and 
a  trifle  further  away  it  will  move  witli,  the  mirror. 

Hypermetropic  Point  of  Keversal. — Rays  of 
light  emerging  from  a  hypermetropic  eye  in  a 
state  of  rest  are  divergent,  and  will  therefore 
never  meet  and  cross.  And  in  this  case  it  will 
require  a  convex  lens  of  more  than  1  D.  to  bring 
these  divergent  rays  to  a  focus  at  1  meter.  It 
will,  in  fact,  need  a  convex  lens  precisely  as  much 


86 


EEFRACTION 


in  excess  of  1  D.  as  the  patient's  eye  is  hyper- 
metropic, in  order  to  bring  the  point  of  reversal  at 
1  meter  where  the  observer's  eve  is  located. 


Illustrates  how  convergent  rays  from  a  slightly  my- 
opic ey.e  are  still  uncrossed  ^vhen  they  reach  the  retin- 
oscope,  causing  the  shaflo\v  to  move  against  tlie  mir- 
ror. Observe  that  tliese  rays  would  meet  at  a  point 
within    infinity. 

Myopic  Point  of  Eeversal. — Rays  of  light 
emerging  from  a  myopic  eye  in  a  state  of  rest  are 
convergent,  and  will  meet  at  a  point  somewhere 


Sliowing  hoAv  sligiitly  convergent  rays  from  the 
slightly  myopic  eye  are  broviglit  to  a  focus  at  1  m?ter 
by  a  convex  lens  as  much  less  than  1  D.  as  the  rays 
are   convergent. 

inside  of  infinity,  i.  e.,  somewhere  within  six 
meters  of  the  patient's  eye.  If  the  degree  of 
mj^opia  present  is  just  1  D.  the  emerging  rays 
will  meet  at  just  1  meter,  i.  e.,  just  where  the  ob- 


HETINOSfOl'Y  87 

servers  e3'e  is  situated;  if  it  is  less  than  1  1). 
they  w  ill  ineet  at  some  point  further  than  1  meter, 
and  will,  tlierefore,  be  still  uncrossed  when  they 
reach  the  observer;  if  it  be  more  than  1  D.  the 
i-ays  Avill  have  met  and  crossed  before  they  reach 
the  observer,  and  the  shadow  will  therefore  moye 
with  the  mirror. 

In  the  first  instance  (myopia  of  just  1  D.),  no 
lens  of  any  kind  will  be  necessary  to  bring  the 


Showing-  how  convergent  rays  from  an  eye  that  is 
myopic  more  than  1  D.  have  met  and  crossed  before 
tliey  reach  the  retinoscope,  causing  the  shadow  to 
move    with    the    mirror. 

point  of  reversal  at  1  meter;  in  the  second  in- 
stance (myopia  of  less  than  1  D.)  it  will  still 
need  a  convex  lens  to  bring  the  point  of  reversal 
at  1  meter  but  it  will  require  a  convex  lens  to 
of  less  than  1  D. — just  as  much  less  as  the  eye 
is  myopic;  in  the  third  instance  (myopia  of  more 
tlian  1  D.)  it  will  be  necessary  to  delay  the  meet- 
ing of  the  rays  in  order  to  brin^"  the  point  of  re- 
versal at  1  meter,  and  hence  a  concave  lens  will  be 
required  of  just  as  much  strength  as  the  myopia 
is  in  excess  of  1  D. 


88  REFRACTION 

Interpretation  of  Shadow. — If,  then,  wit!) 
the  observer's  eye  at  one  meter  from  the  patient's, 
the  shadow  is  seen  to  move  against  the  mirror, 
the  patient's  eye  is  either  emmetropic  (rays  par- 
allel), or  hypermetropic  (rays  divergent),  or  less 
than  1  D.  myopic   (rays  convergent  but  meeting 


Showing  how  convergent  rays  are  brought  to  a  focus 
at  1  meter  by  a  concave  lens  of  dioptric  strength 
etiuivalent  to  the  excess  of  myopia  over  1  D.  In  this 
illustration  the  dotted  line  represents  the  convergence 
which  would,  unaided,  bring  the  p.  of  r.  at  1  meter, 
i.e.,  1  D.  of  myopia.  A  0.50  D.  concave  lens  render  the 
actual  rays  equal  to  this,  hence  there  is  1.50  D. 
myopia   in    the   eye. 

at  further  than  1  meter) .  If  they  move  with  the 
mirror,  the  eye  is  myopic  more  than  1  D.  (rays 
convergent  and  crossing  in  less  than  1  meter). 

Emmetropia. — ^ow  if  a  convex  lens  of  1  D. 
before  the  patient's  eye  bring  the  point  of  reversal 
just  where  the  observer's  eye  is,  so  that  by  mov- 
ing a  trifle  backward  or  forward  the  shadow  moves 
respectively  with  and  against  the  mirror,  we  know 
that  the  rays  emerging  from  the  eye  are  parallel, 
and  the  refraction  of  the  eye  is  normal. 

Hypermetropia. — If,  with  a  convex  lens  of  1 
D.,  the  shadow  still  move  against  the  mirror,  we 


RETINOSCOPY  89 

know  that  the  1  D.  lens  has  not  succeeded  in  bring- 
ing the  rays  to  a  crossing  point  at  1  meter,  and 
that  they  must  therefore  be  divergent  rays,  i.  e., 
the  eye  is  hypermetropic.  We  then  try  stranger 
convex  lenses  until  we  find  one  which  just  re- 
verses the  movement  of  the  shadow.    The  strength 


Showing-  how  convergent  rays  from  an  eye  that  is 
myopic  just  1  D.  came  to  a  focus  at  just  1  meter, 
bringing  the  p.  of  r.  exactly  at  the  retinoscope.  The 
myopic  effect  of  the  eye  is  just  equivalent  to  that  of 
a  1   D.   convex  lens    (see  above). 


of  this  lens  in  excess  of  1  D.  is  the  measure  of  the 
divergence  of  the  rays,  and  therefore  of  the  hyper- 
nietropia  of  the  eye. 

^Iyofia. — H  on  the  other  hand,  the  shadow  is 
seen  to  move  against  the  mirror,  and  a  convex 
lens  of  1  D.  completely  reverse  the  movement,  we 
know  tliat  the  1  D.  convex  lens  has  brought  the 
rays  to  a  crossing  point  nearer  than  1  meter,  and 
that  they  must  therefore  be  convergent  rays,  i.  c., 
the  eye  is  myopic,  but  less  than  1  D.  myopic.  We 
then  try  weaker  and  weaker  convex  lenses  until 
we  find  one  that  just  reverses  the  movement  of 
the  shadow.     The  strength  of  this  lens  less  than 


00  REFRACTION 

1  D.  is  the  measure  of  the  convergence  of  tlie 
rays,  and  therefore  of  the  myopia  of  the  eye. 

If,  Avith  the  observer's  eye  at  1  meter,  the  image 
is  indistinct,  and  by  moving  a  triflle  backward  and 
forward  the  shadow  is  seen  to  move  respectively 
with'  t^nd  against  the  mirror,  we  know  that  the 
rays  emerging  from  the  eye  are  just  convergent 
enough  to  reach  their  crossing  point  at  1  meter; 
i.  e.,  the  eye  is  just  1  D.  myopic. 

If,  however,  with  the  observers  eye  at  1  meter, 
the  shadow  is  at  once  seen  to  move  with  the  mir- 
ror, we  know  that  the  emerging  rays  have  already 
met  and  crossed  at  some  point  nearer  than  1  me- 
ter; i.  e.,  that  the  eye  is  more  than  1  D.  myopic. 
In  that  case  it  is  necessary  to  delay  their  meeting 
in  order  to  bring  the  point  of  reversal  forward  to 
1  meter,  and  we  tlierefore,  beginning  with  the 
weakest,  try  successively  stronger  concave  lenses 
until  we  find  the  one  that  just  reverses  the  move- 
ment of  the  shadow.  The  strength  of  this  lens 
tells  how  much  in  excess  of  1  D.  the  eye  is  myopic. 

Examples. 
1.  Shadow  Is  Seen  to  Move  Against  the 
Mirror. — Rays  are  either  parallel,  divergent,  or 
so  slightly  convergent  that  they  have  not  yet  met. 
Eye  is  either  emmetropic,  hyperopic,  or  slightly 
myopic.  Convex  lens  of  1  D.  just  brings  the  point 
of  reversal  to  the  observer's  eye,  so  that  by  moving 
slightly  backward  and  forward  the  shadow  is  made 
to  move  respectively  with  and  against  the  mirror. 


RETINOSCOPY  9 1 

The  rays  are  parallel,  and  the  refraction  of  the 
eye  normal. 

2.  Shadow  Is  Seen  to  ]\[ovi-:  x\gainst  the 
Mirror. — With  convex  lens  of  1  L).  they  still  move 
against.  Emerging  rays  are  divergent;  eye  is 
hypermetropic.  Convex  lens  of  2.50  D.  just  brings 
the  point  of  reversal  to  observer's  eye.  The  eye  is 
hypermetropic  1.50  D.  and  a  convex  lens  of  that 
strength  will  correct  the  hypermetropia. 

3.  Shadow  Is  Indistinct  With  the  Naked 
Eye. — By  moving  a  trifle;  backward  and  forward 
the  observer  sees  it  move  respectively  with  and 
against  the  mirror.  The  point  of  reversal  is  ex- 
actly at  observer's  eye.  Emerging  rays  are  just 
sufficiently  convergent  to  meet  at  one  meter ;  hence 
the  eye  is  just  1  D.  myopic. 

4.  ^  Shadow  Is  Seen  to  Move  Against  the 
Mirror.— With  convex  lens  of  1  D.  it  moves  de- 
cidedly with  the  mirror.  Emerging  rays  are  con- 
vergent, not  sufficiently  so  to  meet  unaided  at  1 
meter,  but  sufficiently  so  that  the  1  D.  convex  lens 
brings  them  to  focus  sooner  than  1  meter.  Convex 
lens  of  0.50  D.  just  brings  the  point  of  reversal 
at  observer's  eye.  The  eye  is  myopic  0.50  less  than 
1  D.,  i.  c.,  0.50  D.  and  a  concave  lens  of  that 
strengili  will  correct  the  myopi'a. 

^5.  Shadow  Is  at  Once  Seen  to  Move  with 
THE  Mirror. — Emerging  rays  have  already 
crossed;  they  are  convergent  to  a  greater  degree 
than  1  D.  and  the  eye  is  myopic  more  than  1  D. 
It  is  necessary  to  delay  their  meeting  to  bring  for- 


92  REFRACTION 

ward  the  crossing  point.  A  concave  lens  of  1.2o 
D.  just  brings  the  point  of  reversal  at  observers 
eye.  The  e3'e  is  myopic  1.25  more  than  1  D.,  i.  e.^, 
2.25  D.,  and  a  concave  lens  of  that  strength  will 
correct  the  myopia. 

Practical  Points. 

Position-  of  Light. — The  best  position  for  the 
light  will  be  found  to  be,  as  already  described, 
about  six  inches  in  front  and  to  the  left  of  the 
observer,  so  that  the  rays  may  cross  obliquely  in 
front  of  his  left  eye.  leaving  it  in  shadow,  and 
fall  obliquely  upon  the  mirror  held  before  liis 
right  eye.  The  observer's  right  eye  is  used  for  ex- 
amination of  both  of  the  patient's  eyes. 

Some  operators,  however,  prefer  to  have  the 
light  a  little  behind  and  above  the  patient's  head, 
because  it  gives  less  annoyance  to  the  unused 
^ye.  It  is  a  mere  matter  of  taste,  but  manifestly, 
he  nearer  the  source  of  light  is  to  the  mirror,  the 
more  brilliant  will  be  the  illumination  of  the  pa- 
tient's pupil. 

Concentration  of  Light. — It  should  be  re- 
membered that  it  is  the  patient's  pupil,  and  the 
pupil  only,  that  is  under  examination.  The  move- 
ments of  the  shadow  on  the  face  and  on  the  cornea 
are  valueless  and  only  serve  to  mislead.  Hence, 
within  certain  limits,  the  smaller  and  more  con- 
centrated the  light  that  is  thrown  into  the  eye 
the  better.  The  average  observer  will  find  that 
a  10  mm.  hole  in  the  diaphragm  of  the  lamp  will 


UETINOSCOPY 


93 


o-ive  him  the  best  area  of  illumination  at  one 
meter.  The  power  of  the  light  must  be  at  least 
?j2  candle  power;  64  c.  p.-is  better. 

Movements  of  Mirkok.— For  the  same  reason, 
namely,  that  the  pupil  is  the  only  part  to  be  ex- 
amined, the  tilting  of  the  head  and  mirror  from 
side  to  side  to  produce  the  movements  of  the 
shadow  shonld  be  very  slight,  so  as  not  to  get  the 
illumined  disc  outside  the  pupil.  When  the  point 
of  reversal  has  almost  been  found,  many  operators 
then  change  to  a  smaller  hole  in  the  diaphragm, 
so  as  to  get  a  still  more  concentrated  illumination 
for  determining  the  last  few  degrees  of  error. 

Focal  Length  of  Mirror. — It  is  important 
that  the  observer  bo  situated  further  from  the  pa- 
tient than  the  focal  length  of  the  mirror,  else  it  is 
evident  that  the  rays  of  light  thrown  by  the  mirror 
into  the  patient's  eye  will  not  have  crossed  be- 
fore they  enter  it,  the  image  made  on  the  pa- 
tient's retina  will  be  an  erect  instead  of  an  in- 
verted one,  and  the  whole  process  will  be  reversed. 
This  can  be  ensured  by  employing  either  a  plane 
mirror,  or  one  of  not  more  than  25  cm.  focal 
length  f(U-  one  meter  -vork. 

•Six  Meter  Method. — Some  operators  prefer  to 
work  at  infinity,  i.  e.,  to  place  themselves  at  six 
meters  from  the  patientV  eye.  This  method  has 
the  advantage  that  no  a  iificial  forcing  of  the 
point  of  reversal  has  to  b«  effected  in  order  to 
make  a  normal  starting  poini",  as  in  the  one  meter 
niclhofl.     Witli  llio  mirror  a-    six  meters  emerging 


94  REFRACTION 

parallel  and  divergent  rays  have  not  met  by  the 
time  they  reach  the  observer^  and  convergent  rays 
have  always  crossed;  hence  the  shadow  always 
moves  against  in  emnietropia  and  liypcropia^  and 
with  in  any  degree  of  myopia. 

The  convex  lens  that  reverses  the  movements- 
of  the  shadow  is  the  straight  measure  of  the 
hypermetropia  and  the  concave  lens  that  accom- 
plishes the  same  effect  is  the  straight  measure  of 
tlie  myopia.  The  method  has  many  disadvan- 
tages, however,  chief  among  which  are  the  diffi- 
culty in  arranging  the  light  satisfactorily,  and  the 
inconvenience  of  being  obliged  to  walk  to  and 
fro  every  time  it  is  necessary  to  change  the  lens 
at  the  patient's  eyes. 

Computing  the  Correction. — With  the  one 
meter  method  it  is  only  necessary  to  remember  that 
from  the  result  obtained,  i.  e.,  from  the  lens  by 
which  the  point  of  reversal  was  found,  -[-  1  I^- 
must  be  subtracted,  that  being  the  refraction  nec- 
essary to  find  the  normal  point.  Thus  if  the  net 
lens  refraction  used  to  find  the  point  of  reversal 
be  —  2  D.,  then  the  necessary  correction  is  —  2 
D.  less  -f-  1  D.,  which  is  —  3D.  If  the  net  re- 
fraction used  be  -f-  2  D-?  f^^^i^  ^^^^  proper  correc- 
tion is  +  2  D.  less  +  1  D.,  which  is  +  1.  D. 

Edges  of  Shadow. — The  clearer  defined  the 
edges  of  the  shadow,  and  the  quicker  it  moves 
across  the  field,  the  lower  the  degree  of  refractive 
error.  Ill-defined,  slowly-moving  shadows  denote 
n  high  degree  of  ametropia.    Note  how  the  shadow 


RE'liNOSCOi'Y  1)5 

becomes  sharper  and  moves  more  rapidly  as  the 
correcting  Ions  is  more  and  more  nearly  ap- 
proached. 

Retinoscopy  in  Astigmatism. 
Althoiigli   the  estimation  of  spherical  errors  of 
refraction    is    the    principal"  object    of    skiaskopic 


Illustrating-  why  the  shadow  in  retinoscopy  is  oblique 
in  astigmatism,  viz.,  because  the  image  is  oval  shaped. 

examination,  these  are  not  the  only  forms  of  error 
that  this  test  enables  ns  to  detect  and  measure. 
Astigmatism  also  shows  up  under  the  shadow  test 
with  more  or  less  definiteness. 

Oblique  Edges. — If  the  eye  is  astigmatic  the 
edges  of  tlie  shadow  made  by  rotating  the  mirror 
from  side  to  side  are  usually  oblique,  no  matter 
how  the  mirror  is  tilted.  This  is  due  to  the  oval 
shape  of  tlie  image  u])on  the  retina.  If  then  the 
edges  appear  oMiciue  it  may  1)0  at  once  concluded 
that  the  eye  is  astigmatic. 

Method  of  Estoiatiox. — If  the  oblique-edged 
shadow  moves  against  tlie  miiTor  the  astigmatism 


96  BEFEACTION 

is  hypermetropic;  if  with  the  mirror_,  it  is  myopic. 
The  mirror  must  then  be  rotated  at  right  angles 
and  parallel  to  the  two  edges  respectively,  and 
csicb.  meridian  corrected  separately^,  the  same  as 
in  hypermetropic  and  myopia.  More  detailed  in- 
structions for  this  will  be  given  when  we  come  to 
consider  astigmatism. 

Varying  Velocity. — Sometimes,  however,  the 
edges  of  the  shadow  are  vertical,  and  move  hori- 
zontally, and  the  only  way  we  are  led  to  suspect 
astigmatism  is  because  one  edge  is  more  distinct 
and  moves  more  quickly  than  the  other.  For  this 
reason  we  must  never  be  content  in  performing 
skiaskopy  to  simply  find  the  point  of  reversal  by 
one  angular  rotation  of  the  mirror  only,  or  we 
may  overlook  an  astigmatism.  The  point  of  re- 
versal having  been  found  by  rotating  the  mirror 
liorizontally,  it  should  then  be  tested  by  rotating 
the  mirror  vertically,  i.  c.,  on  a  horizontal  axis, 
and  at  one  or  two  other  angles,  in  order  to  make 
sure  that  the  refraction  is  the  same  in  all  meri- 
dians. 

We  shall  refer  to  this  whole  matter  at  more 
length  in  a  future  chapter  on  Astigmatism. 


CHAPTER  VIII. 
OPHTHALMOSCOPY. 

Ophthalmoscopy  is  considerably  more  difficult 
to  carry  out  than  retinoscopy,  and  it  is  question- 
able whether  as  valuable  or  as  accurate  informa- 
tion is  obtained  from  it.  Nevertheless,  it  has  its 
place  in  refraction,  and  should  be  practiced  in 
every  case,  especially  as  by  its  means  a  patho- 
logical condition  of  the  eye  is  often  detected 
wliich  would   otherwise   escape  attention. 

The  ophthalmoscope  is  in  principle  a  simple  in- 
strument, being  essentially  nothing  more  than  a 
small  plane  mirror,  similar  to  a  retinoscope,  but 
a  little  smaller,  mounted  on  a  short  stem  handle, 
and  provided  with  a  central  sight  hole.    However,- 
the  various  modern  makes  of  ophthalmoscope  are 
fitted  witli  a  series  of  convex  and  concave  lenses 
which  are  made  to  revolve  by  means  of  a  wheel 
at  the  back   of   the   mirror,   so  tliat  any  diopter 
lens,  plus  or  minus,  can  be  wheeled  in  front  of 
the   observer's   eye,   and   thus   obviate   the   rather 
cumbersome  necessity   of   luounting  lenses  in  the 
spectacle  frame  during  this  examination. 

For  the  satisfactory  accomplishment  of  ophthal- 
moscopy it  is  ahnost  indispensable  to  employ 
atropin,  or  a  convex  k'us,  for  the  reflection  from 
tlie  mirror  is  exceedingly  bright  (the  instrument 
being  lu>ld  very  close  to  the  eye),  and  complete 
relaxation  of  the  acconinuxlation  is  a])solute]y 
essential  to  correct  I'csults. 


d8 


REFRACTION 


Ophthalmoscopy    is    divifled    into    two    general 
methods,  the  (lirect  and  indirect. 


The    Ophthalmoscope. 


Direct  Ophthalmoscopy. 
The    Principle.— Tl;e    essential    principle    of 


OPHTHALMOSCOPY  99 

oplitlialmoscop}'  is  the  same  as  that  of  rctinoscopy, 
namel}-,  the  bringing  to  focus  of  tlie  rays  that 
emerge  from  ilie  patient's  eye.  However,  in 
letinoscopy  the  object  of  the  test  is  to  bring  them 
to  focus  at  a  certain  known  distance  in  front 
of   the  patient's   eye,   and    as   tliis  distance    (one 


OISbKRVEK  S    KYK  I'ATIENT's    EYK 

Illustrates  huw,  in  direct  opthal,  parallel  rays  from 
a  normal  eye  at  rest  are  focused  on  tlie  retina  of 
the  observer's  normal   relaxed  eye. 

meter)  is  too  great  to  permit  of  the  observer  get- 
ting an  intelligible  image  of  the  patient's  retina 
upon  his  own,  even  when  the  rays  have  been 
properly  focused,  he  is  obliged  to  depend  upon 
the  movements  of  the  image  and  its  shadow  to 
tell  when  the  focus  has  heen  found. 

In  direct  ophthalmoscopy  the  object  is  to  bring 
the  rays  that  emerge  from  the  patient's  eye 
directly  to  a  focus  on  the  retina  of  the  observer 
by  rendering  Jjotli  the  observer's  eye  and  the 
patient's  eye  perfectly  normal  as  to  refraction 
and  relaxed,  as  to  acconiniodation  ;  and  the  evi- 
dence that  tin's  has  been  accomplished  is  in  the 
perception  by  the  observer  of  a  clear  image  of 
the  patient's  retina  or  fundus. 


100  REFRACTION 

If  the  patient^s  eye  is  normal  in  refraction  and 
completely  relaxed  as  to  accommodation,  then  the 
rays  that  emerge  from  it  are  parallel;  and  if  the 
observer's  eye  is  similarly  normal  and  relaxed, 
then  it  is  adapted  to  receive  and  focus  parallel 
rays;  hence  the  details  of  the  patient's  fundus 
will  be  plainly  pictured  on  the  abservers  retina, 
and  clearly  seen  by  him. 

We  start  out  by  taking  care  that  the  observer's 
eye  is  normal  in  refraction  and  relaxed  as  to  ac- 
commodation; we  then  see  that  the  patient's  eye 
is  relaxed  as  to  accommodation,  either  by  paralyz- 
ing the  accommodation  or  having  him  relax  it; 
then  the  only  remaining  factor  in  the  equation  is 
the  patient's  refraction — if  this  be  normal  the 
two  opposing  eyes  will  be  identically  alike,  rays 
omero-inff  from  the  one  will  be  accuratelv  focused 
on  the  other,  and  the  observer  will  see  the  fundus 
clearly ;  but  if  it  be  abnormal,  so  that  rays  emerg- 
ing from  the  patient's  e3^e  are  either  divergent  or 
convergent,  then  in  order  to  focus  them  on  the 
observer's  retina  a  convex  or  concave  lens  will  be 
required  equal  to  the  degree  of  divergence  or  con- 
vergence in  question,  i.  e.,  equal  to  the  hyperopia 
or  the  myopia  of  the  patient's  eye. 

Technic  of  Direct  ]\ri:TTiOD. — In  the  direct 
method,  the  light,  which  shonld  l)e  similar  to  that 
used  in  retinoscopy,  is  placed  at  the  side  of  tlie 
patient's  h(>ad,  on  tlie  same  side  as  the  eye  to  be 
examined,  about  on.  a  level  Avith  his  temple,  so 
that  no  direct  rays  fall  on  his  eye.     If  the  opera- 


OPHTHALMOSCOPY 


101 


tor  liavu  any  error  of  refraction  he  must  correct 
it  with  }3roper  glasses.  The  operator  seats  him- 
sc'ir  i  111  mediately  facing  the  patient^,  on  the  same 
side  as  the  eye  under  observation^  and  uses  the 
same  relative  eye  hims(>lf.     That  is,  in  examining 


IHustrales    the    technic    of    direct    oplithalmoscopy 


llie  patient's  right  eye,  he  sits  on  his  right  side, 
and  uses  his  own  right  eye.  The  observer's  un- 
used eye  should  remain  open  during  tlic  observa- 
tion. 

The  Oj)cral(ir,  liolding  llic  iniri'or  about  1-")  cm. 
from  tlie  patient's  eye,  and  relaxing  his  own  ac- 
commodation,  applies  his  eye  to  the  sight  hole 


102  REFRACTION 

and  throws  tlie  rclleclecl  light  into  tho  patient's 
pupil.  As  soon  as  the  red  fundus  reflex  is  ob- 
tained, the  operator,  with  his  eve  at  the  sight 
hole,  gradually  approaches  nearer  and  nearer  to 
the  patient's  eye,  taking  care  to  keep  his  own  ac- 
commodation relaxed  and  to  keep  the  light  focused 
on  the  patient's  pupil,  until  he  is  quite  close  to 
the  ])atient's  eye. 

Observation  of  Fundus. — If  the  patient's  re- 
fraction is  normal,  the  details  of  the  fundus,  i.  e., 
the  blood  vessels,  etc.,  should  now  be  plainly  seen, 
for  if  the  patient's  eye  is  completely  relaxed,  the 
rays  of  light  leaving  it  are  parallel,  and  if  the 
observers. eye  is  similarly  relaxed  it  is  in  a  posi- 
tion to  receive  and  focus  parallel  rays,  hence  the 
details  of  the  patient's  fundus  should  be  plainly 
pictured  on  the  observer's  retina. 

Possible  Errors. — If  at  the  first  attempt  the 
fundus  is  not  clearly  seen,  the  observer  should 
try  two  or  three  times,  in  order  to  make  sure 
that  it  is  not  his  own  accommodation  or  his 
technic  at  fault.  The  operation  is  one  which  re- 
quires considerable  practice  and  cannot  be  done 
hurriedly,  but  the  results  from  a  scientific  point 
of  view  are  well  worth  trying  for.  Beginners  find 
it  especially  hard  to  keep  their  accommodation 
relaxed.  This  can  most  easily  be  done  by  imagin- 
ing the  patient's  fundus  to  be  away  at  the  back 
of  his  head^  «and  gazing  into  it  with  that  impres- 
sion. 


OlMrrilALMOSCOl'Y 


103 


Use  of  the  Ophthalmoscope. 
In  AiMETKorjA.— When  quite  conviuwd  that  his 
own  technic  or  accommodation  is  not  at  fault, 
;in(l  the  imago  of  the  patient's  fundus  still  re- 
jiiai us  indistinct,  tlie  only  conclusion  for  the  ob- 
server to  come  to  is  tliat  tlie  rays  emerging  from 
\ho  patient's  eye  are  not  parallel — they  are  either 
(li\i"rgcnt  (liypcrniefroi)ia)  or  convergent  (myopia) 


CONVEX    LENS 


0B8ER  VEn'S  EYE 


PATIENTS   EVC 


IHustrates  how,  in  direct  ophthalmoscopy,  divergent 
rays  from  the  hyperopic  eye  are  focused  on  the  ob- 
server's retina  (i.  e.,  rendered  equivalent  to  parallel 
rays)  by  a  convex  lens  equal  to  the  degree  of  di- 
vergence. 


and  it  will  be  necessary  to  interpose  a  lens,  con- 
vex or  concave  as  the  case  may  be,  whose  dioptric 
strength  just  equals  the  degree  of  divergence  or 
f  onvcrgence  of  the  patient's  rays,  in  order  to  focus 
lliein  on  the  observers  retina.  Whetlier  this  lens 
shall  be  convex  or  concave,  and  of  what  strength, 
can   only  l)e  ascertained  ])y  trying. 

Ty  llvi'i:i{Mi:'n:()i'i.\. — The  observer  should  first 
wheel  a  weak  convex  lens  in  fi-ont  of  the  sight 
liole  of  the  instrument,  and  look  again.  If  this 
helps,  he  should  wheel  successively  stronger  con- 


104 


REFRACTION 


vex  lenses  in  front  until  he  finds  the  strongest 
convex  lens  that  gives  him  a  clear  view  of  the 
details  of  the  fundus.  This  indicates  that  the 
patient  is  hypermetropic,  i.  e.,  the  rays  emerging 
from  his  eye  are  divergent,  and  the  lens  which 
focused  them  on  the  observer's  retina,  i.  e.,  which 


OBSEB  VERS  EVE- 


lUustrates  how  convergent  rays  from  a  myopic  eye 
are  focused  on  the  observer's  retina  (1.  e.,  rendered 
equivalent  to  parallel  rays)  by  a  concave  lens  equal 
to  the  degree  of  convergence    (myopia). 


made  them  equivalent  to  parallel  emerging  rays, 
is  the  measure  and  correction  of  the  hyperme- 
tropia. 

In"  Myopia. — If  a  weak  convex  lens  makes  the 
fundus  still  more  indistinct,  the  observer  should 
then  try  a  concave  lens,  and  if  this  helps  he 
should  wheel  successively  stronger  concave  lenses 
before  the  sight  hole  until  he  finds  the  weakest 
concave  lens  which  gives  a  clear  view  of  the  fundus. 
This  indicates  that  the  patient  is  myopic,  i.  e., 
that  the  rays  emerging  from  his  eye  are  converg- 
ent, and  the  lens  wliich  focused  them  on  the  ob- 
server's retina,  i.  e.,  which  made  them  equivalent 
to  parallel  emerging  rays,  is  the  measure  and  cor- 
rection of  the  myopia. 


OPHTHALMOSCOPY  105 

In  Astigmatism. — As  in  retinoscopy,  so  in  di- 
rect ophthalmoscopy,  astigmatism  may  be  detected 
v.'ith  some  degree  of  definiteness.  If  the  patient's 
eye  is  astigmatic,  it  is  evident  that  the  details  of 
the  fundus  will  be  distinctly  visible  in  one  me- 
ridian while  they  are  indistinct  or  invisible  in 
another. 

In  this  case  the  operator  finds  the  strongest 
convex  or  the  weakest  concave  spherical  lens  which 
enables  him  to  gain  a  clear  view  of  the  most 
indistinct  meridian,  and  this  spherical  lens  is  the 
measure  of  the  convex  or  concave  cylinder  which 
will  correct  the  astigmatism. 

For  reasons  already  explained,  the  axis  of  the 
correcting  cylinder  must  be  placed  at  right  angles 
to  the  defective  meridian.  More  detailed  instruc- 
tions for  this  ratlier  difficult  operation  will  be 
found  in  tlic  chapter  on  astigmatism. 

Observer's  Kefraction. — The  results  of  the 
direct  metliod  of  ophtlialnioscopy,  while  not  equal 
to  those  of  retinoscopy,  are  yet  very  valuable  and 
accurate  when  the  process  is  properly  carried  out, 
and  like  retinoscopy,  it  has  the  advantage  of  being 
quite  objective  in  character.  Naturally,  however, 
the  success  of  tlie  examination  depends  entirely 
ii])(tn  llic  observer  liiiusclf.  'Vho  fii'st  thing  lie 
Dinst  be  careful  of  is  tliat  liis  nw n  refraction  is 
normal.  If  therefore  he  has  any  error  of  refrac- 
tion it  is  imperative  that  lie  obtain  and  wear  dur- 
ing the  observation  a  proper  correction  of  his  own 
ametropia.     Or,  if  he  prefers  to  work  without  his 


106  REFRACTION 

own  correction,  he  must  at  least  know  just  what 
his  own  error  is,  and  make  allowance  for  it  in  esti- 
mating the  result  of  his  observation,  adding  it  to 
or  subtracting  it  from  the  lens  power  needed  to 
focus  the  patient's  retina  on  his  own,  as  the  case 
may  be. 

Observer's  Accommodation. — The  other  prin- 
cipal factor  in  the  success  of  tlie  examination 
is  that  the  observei-'s  accommodation  shall  be  re- 
laxed during  the  observation,  else  of  course  par- 
allel rays  from  the  patient's  eye  will  not  focus 
on  his  retina.  This  is  by  far  the  most  difficult 
item  in  the  whole  procedure,  and  is  usually  only 
acquired  by  long  practice.  Some  operators  find 
it  helpful  to  wear  an  opaque  disc  or  a  strong- 
convex  lens  over  the  unused  eye  during  exam- 
ination, so  as  to  induce  relaxation  of  the  eye 
they  are  using;  if  this  is  done,  however,  it  is  nec- 
essary to  use  a  very  thin,  closely-fitting  mounting 
frame,  so  as  not  to  interfere  witli  tlie  hohling 
of  the  ophthalmoscope  at  the  active  eye. 

Allowance  for  Atropin  or  Lens. — If  atropin 
or  a  convex  lens  has  been  employed  to  paralyze  the 
patient's  accommodation,  an  allowance  of  1  D. 
must  be  made  for  the  former  in  estimating  the 
final  result,  or  of  the  actual  strength  of  the  latter. 
Indirect  Ophthalmoscopy. 

The  Indirect  MctJtod  of  ophthalmoscopy  is  still 
more  diflicult  of  accomplishment,  and  more  meager 
in  results  than  the  direct  method. 

The   Principle   of   the    indirect   method    more 


OPHTHALMOSCOPY  107 


nearly  rot^einbk'S  that  of  retiiioscopy ;  that  is  to 
sav,  it  consists  in  an  artificial  forcin^r  of  the  focus- 


Sliowing  how,  in  indirect  ophthalmoscopy,  i)aralh'l 
rays  from  an  emmetroi)ic  eye  are  focused  by  the  objec- 
tive lens  the  same  distance  beyond  it,  no  matter  how 
far  from  the  eye  the  objective  is  held,  line  A  B  equal 
C  D.      (Image  remains  same  size.) 

ing  of  the  rays  which  emerge  from  the  patient's 
e3-e.  By  means  of  a  strong  convex  lens  held  close 
in  front  of  the  patient's  eye  the  emerging  rays  are 
made  to  meet  and  cross  a' very  short  distance  in 


Showing  how  divergent  rays,  in  indirect  ophthalmos- 
copy from  the  hyperopic  eye  are  focused  further  be- 
yond the  objective  the  further  away  from  the  eye  the 
objective  is  held.  C  D  greater  than  A  B.  (Image  gets 
smaller   and    smaller.) 

front  of  the  eye,  and  the  observer's  eye  is  held 
sufficiently  far  from  the  patient  that  he  receives 


108  REFRACTION 

a  very  enlarged  picture  of  a  very  small  area  of 
the  patient's  fundus.  As  it  is  necessary  that  this 
small  area  be  definitely  circumscribed,  the  disc 
is  chosen  as  the  point  of  observation. 

As  the  emerging  rays  have  crossed  long  before 
thev   reach   the   observer's   eye,   he   sees   a    re-in- 


Showing  how,  in  indirect  ophthalmoscopy,  converg- 
ent rays  from  a  myopic  eye  are  focused  less  and  less 
beyond  the  objective,  the  further  from  the  eye  the  ob- 
jective is  held,  C  D  less  than  A  B.  (Imag-e  gets  bigger 
and  bigger.) 

verted,  that  is,  an  aerial  image,  formed  between 
the  lens  and  his  eye.  This  aerial  image  must  now 
be  regarded  as  the  object  at  which  the  observer  is 
looking. 

Now,  if  the  rays  emerging  from  the  patient's 
eye  are  parallel  (emmetropia),  no  matter  at  what 
distance  the  convex  lens  is  held,  these  parallel 
rays  will  focus  at  the  same  distance  from  the 
lens,  namely,  at  its  principal  focal  distance,  and 
the  size  of  the  image  will  bo  the  same. 

If  the  emerging  rays  are  divergent  (livper- 
metropia),  the  further  away  the  lens  is  moved 
from  the  point  of  emergence  the  more  divergent 
the  rays  will  be  when  they  striken  the  lens,  the 


OPHTHALMOSCOPY 


109 


longer  they  will  take  to  focus,  the  image  will  be 
formed  further  and  further  beyond  the  lens,  and 
will  consequently  be  smaller  and  smaller. 

On  the  other  hand,  if  the  emerging  rays  are 
convergent  (myopia),  the  further  away  the  lens 
is  moved  from  the  point  of  emergence  the  more 
convergent  the  rays  will  be  when  they  strike  the 


^^^SJ'^s 


Illustrates  the  technic  in  indirect  ophthalmoscopy. 


lens,  the  more  quickly  they  will  focus,  the  image 
will  be  formed  less  and  less  distance  beyond  the 
lens,  and  will  therefore  be  larger  and  larger. 

Technic  of  Indirect  Method, — The  light  and 
general  arrangement  are  the  same  as  in  the  di- 
rect method.  In  this  case,  however,  the  ophthal- 
moscope, with  the  observer's  63^6  at  the  sight  hole, 
is  maintained  at  about  33  cm.  from  the  patient's 
eye,  and  the  observer  can  use  the  same  eye  in 
examining  both   those  of  the  patient.      A   strong 


110  REFRACTION 

convex  lens  is  then  held  by  the  observer's  unoc- 
cupied hand  close  against  the  patient's  eye^  in 
sucli  a  way  as  not  to  obstruct  the  light  from  the 
lamp  to  the  mirror.  One  of  these  strong  convex 
lenses  is  included  in  the  ophthalmoscope  case,  but 
the  writer  has  usually  found  it  too  strong,  and 
prefers  a  9  D.  or  12  D.  from  the  trial  case. 

View  of  the  Disc. — As  already  intimated,  in 
this  method  of  examination  it  is  not  sufficient  to 
get  a  view  of  the  fundus  of  the  eye.  Having 
thrown  the  light  into  the  pupil  through  the  strong 
convex  lens  (called  the  objective),  and  obtained 
the  red  fundus  reflex,  the  mirror  must  be  focused 
so  as  to  reveal  the  disc  of  the  e3^e.  This  re- 
quires considerable  practice,  and  can  at  first  only 
be  accomplished  by  simply  shifting  the  mirror 
until  the  disc  comes  into  view.  The  observer  will 
know  when  he  has  focused  the  disc  by  the  fact 
that  the  red  fundus  reflex  will  change  to  a  glisten- 
ing buff  color. 

This  image  of  the  disc  (which,  being  viewed 
through  a  couatx  lens,  is  a  virtual  not  a  real 
image),  must  be  kept  steadily  in  view  and  the 
objective  lens  slowly  withdrawn  in  a  perfectly 
straight  line,  away  from  the  patient's  eye  toward 
tlie  observer's. 

Use  of  the  Ophthalmoscope  (Indirect  Method). 
In   Emmetropia. — If  upon  withdrawal  of  the 
objective  the  image  of  the  disc  remains  the  same 
size,  the  eye  is  normal. 


OPHTHALMOSCOPY  111 

In  Hypermetropia. — If  the  image  diminish  in 
size^  tlie  eye  is  hxpermotropic.  The  greater  the 
diminution  the  higher  the  degree  of  hyperme- 
tropia. Convex  lenses  shoukl  tlien  he  wheeled  be- 
fore the  siglit  liole  nntil  the  strongest  one  is 
found  wliicli  will  cause  the  image  to  remain  sta- 
tionary in  size  on  withdrawing  the  objective.  This 
is  the  measure  of  the  liypcrmetropia. 

In  Myopia.— If  the  image  increases  in  size,  the 
eye  is  myopic;  the  greater  the  increase  the  higher 
tlie  degree  of  myopia.  Concave  lenses  must  then 
be  wdieeled  before  tlie  sight  liole  until  the  weak- 
est is  found  wliicli  will  neutralize  the  change  in 
the  size  of  the  image.  'I'll is  is  the  measure  and 
.correction  of  the  myopia. 

In  AsTJGMATis:\r. — If  the  image  increase  or  di- 
minish 'in  size  in  one  direction  only,  the  eye  is 
astigmatic  in  the  meridian  at  riglit  angles  to  the 
direction  in  which  the  image  changes.  If  it  di- 
minish the  astigmatism  is  hypermetropic,  if  it 
increase  it  is  myopic.  And  that  spherical  lens, 
wheeled  before  the  sight  hole,  Avhicli  neutralizes 
it  is  the  measure  of  the  cylinder  which,  when 
placed  with  its  axis  at  right  ansfles  to  the  defect- 
ive meridian,  w'ill  correct  the  astigmatism. 

The  carrying  out  of  this  method  of  ophthal- 
moscopy is  exceedingly  difTicult,  and  requires  long 
and  careful  practice.  In  the  writers  judgment 
it  is  neitlier  so  reliable  nor  so  valuable  as  rhinos- 
copy. 


CHAPTER  IX. 

CORRECTION  OF  HYPERMETROPIA. 

llypermetropia  has  already  been  described  as 
that  condition  of  the  eye  wherein  parallel  rays 
are  bronglit  to  focus  behind  the  retina.  In  other 
words,  the  antero-posterior  diameter  of  the  eye 
is  too  short  in  ])roportion  to  the  refracting  power 


V 


Showing-  the  effect  of  chromatic  refraction  in  the 
hvperopic  eye.  The  light  lines  represent  the  blue  rays 
AA,  the  heavy  lines  the  red  rays  BB.  The  former 
are  nearer  to  focus  at  the  retina  than  the  latter,  giv- 
ing a  blue  centre,   surrounded   by  the  red. 

of  the  eye,  and  the  principal  focal  point  is  situ- 
ated back  of  the  retina. 

Chromatic  Test. — In  the  trial  case,  if  it  be  a 
complete  one,  will  be  found  what  is  termed  a 
chromatic  lens.  This  lens  is  a  combination  o^ 
cobalt  lenses  so  arranged  as  to  suppress  all  of  the 
rays  passing  through  them  except  the  violet  an. 
the  red.  The  test  is  based  upon  the  physical  la\^ 
that  the  liigli-velocity  waves,  i.  e.,  the  violet,  art 
refracted  to  a  greater  degree  llinn  tlie  low-veloc 
ity,  1.  0.,  the  red. 


114  REFRACTION 

Now  if  the  refraction  of  the  eye  be  normal,  the 
difference  in  the  refraction  of  the  two  types  of 
rays  will  not  be  sufficiently  marked  to  produce 
any  sensible  chromatic  aberration^  and  the  patient 
will,  upon  looking  at  a  flame  through  the  lenS;,  see 
an  ordinary  white  flame.  If,  however,  his  eye  is 
hyperopic,  the  violet  rays  have  not  yet  come  to 
a  focus  by  the  time  they  reach  the  retina,  the  red 
rays  are  still  further  from  focusing,  hence  the  ap- 
pearance of  the  flame  is  a  violet  center  with-  a 
red  margin.  The  effect  of  the  cobalt  upon  the  vio- 
let rays  is  to  make  them  appear  blue;  so  that,  as 
a  matter  of  fact,  the  hypermetropic  eye  sees  a  red 
margin  ajound  a  blue  center.  In  other  refractive 
errors  the  chromatic  lens  gives  other  appearances 
whieh  will  be  described  under  their  proper  cap- 
tions. The  value  of  this  test  is  of  course  limited 
to  the  mere  detection  of  the  error;  it  affords  no 
basis  for  its  estimation  or  correction. 

Distant  Type  Test. — The  principle  of  the  dis- 
tant type  has  already  been  explained.  It  con- 
sists of  uniform  black  letters,  so  constructed  that 
their  horizontal  and  vertical  dimensions  subtends, 
at  the  distance  at  which  they  are  designed  to  be 
used,  the  minimum  visual  angle.  That  is  to  say, 
two  lines  drawn  from  the  extreme  boundaries  of 
the  letters  marked  6,  at  a  distance  of  6  meters 
from  the  eye,  through  the  nodal  point,  will  make 
with  each  other  an  angle  of  1  deg.,  which,  as 
previously  explained,  is  the  minimum  angular  dis- 
tance at  which  two  points  can  be  distinguished. 


CORKECTIOJN    OF    llYrEKiMli^TKOri.V  115 

It  cannot  be  too  insistently  borne  in  mind  that, 
while  we  avail  ourselves  of  this  principle  of  acuity 
of  vision  for  making  refraction  tests,  the  actual 
faculty  of  visual  acuteness  is  a  function  of  the 
brain,  differing  in  different  persons  even  when  the 
refraction  is  uniformly  normal,  and  it  is  not  the 
visual  acuity  which  we  are  testing,  but  the  ability 
of  the  eye  to  focus  on  its  retina  the  parallel  rays 
which  proceed  from  the  test  type.  We  make  use 
of  the  minimum  visual  angle  simply  because  it 
affords  the  most  delieate  test  of  refractive  ability. 

If  the  refraction  of  the  eye  is  normal,  then,  it 
ought  to  be  able  to  distinguish  clearly  distant 
type  jSTo.  6  at  a  distance  of  6  meters,  because 
rays  proceeding  from  this  distance  are  parallel 
when  they  reach  the  eye,  and  the  eye  at  rest  is 
normally  able  to  focus  parallel  rays  on  its  retina. 

But  a  hypermetropic  eye  can  also  read  this 
type  at  this  distance,  because,  although  in  a  state 
of  rest  it  is  unable  to  focus  parallel  rays,  it  can 
do  so  by  using  some  of  its  accommodation.  How 
then  shall  we  differentiate  between  the  normal  and 
the  hypermetropic  eye?  By  a  very  simple  and 
logical  test.  Tlie  normal  eye,  reading  No.  6  type 
at  G  meters,  lias  its  accommodation  completely  re- 
laxed, i.  c.,  it  has  assumed  the  least  convex  form 
of  wliicli  it  is  capable.  Thprefore  if  wc  now 
mount  a  woak  convex  Umis  before  it,  thereby  has- 
tening the  focusing  of  the  ra^s  a  little,  the  normal 
eye  Ijas  no  ineans  of  adjusting  itself  to  the  new 
focus,  and  its  vision  is  blurred.     But  the  hyper- 


116  BEFK ACTION 

metropic  eye^,  reading  No.  G  type  at  6  meters,  is 
employing  some  of  its  accommodation;  hence  it 
we  place  a  weak  convex  lens  before  it,  thereby 
hastening  the  focusing  and  bringing  the  focal 
point  forward  a  little,  it  can,  by  relaxing  its  ac- 
commodation, still  read  the  type  quite  well ;  indeed 
it  does  so  with  a  feeling  of  greater  easiness  than 
before. 

The  rule,  then,  is  that  if  the  vision  is  impaired 
by  a  weak  convex  lens  the  eye  is  normal,  if  not 
it  is  hypermetropic. 

Measure  of  Hyperopia. — Now  if  we  keep  try- 
ing stronger  and  stronger  convex  lenses  until  we 
find  the  one  beyond  which  any  further  increase 
in  strength  blurs  the  vision,  we  have  found  the 
convex  lens  which  renders  the  rays  equivalent  to 
parallel  ones,  and  puts  the  eye  in  the  position  of 
a  normal  eye.  This  lens,  then,  is  the  measure  of 
the  hyperopia  of  the  eye,  and  will  be  the  proper 
correction. 

Gradual  Relaxation. — Practically,  however, 
the  best  procedure,  after  once  ascertaining  by  a 
weak  convex  lens  that  the  eye  is  hyperopic,  is  to 
at  once  over-correct  the  error  with  a  strong  con- 
vex lens,  and  then  gradually  reduce  its  strength 
by  mounting  concave  lens  before  it,  until  the  pa- 
tient can  just  read  No.  6  at  6  meters.  The  net 
convex  correction  then  before  the  eye  is  the 
measure  of  its  hyperopia.  The  advantage  of  this 
plan  over  that  of  trying  successively  stronger  con- 
vex lenses  is  thnt  it  first  completely  relaxes  the  cil- 


COKUECTION  OF   llYPEK.METKOriA  117 

iary  iiiuscle,  and  then  gradually  finds  the  point 
where  is  begins  to  contract,  which  is  more  relia- 
ble than  gradually  inducing  it  to  relax. 

Example. — For  example,  suppose  -\-4:  D.  over- 
corrects  the  hypermetropia  and  blurs  the  vision. 
Beginning  witli  — 1  D.,  put  up  in  front  of  the 
-]-4  D.  lens  a  series  of  successively  stronger  con- 
cave lenses  until  No.  6  can  be  read  at  six  meters. 
Suppose  that  — 2  D.  makes  this  possible.  Then 
the  correction  is  +4  D.  less  —2  D.,  that  is,  +2  I). 

Retinoscopy  IX  Hypermetropia. — Under  re- 
linoscopy  the  rays  emerging  from  the  patient's  eye 
are  divergent,  and  will  not  have  met  by  the  time 
they  reach  the  observer's  eye  at  one  meter,  hence 
the  shadow  will  move  against  the  mirror.  Xeither 
would  they  have  met  had  they  been  parallel,  i. 
e.,  if  the  patient's  eye  had  been  normal.  But  if 
they  were  parallel  a  convex  lens  of  1  D.  before 
the  patient's  eye  would  bring  them  to  focus  at 
one  meter,  i.  e.,  at  the  observer's  eye,  and  give 
the  point  of  reversal  at  that  point ;  whereas  if  they 
are  divergent,  a  convex  lens  of  1  D.  will  not  be 
suiticient  to  focus  them  at  the  observer's  eye. 

The  rule,  then,  is  that  if  the  shadow  move 
against  the  mirror,  and  a  convex  lens  of  1  D. 
does  not  give  (lie  point  of  reversal,  the  eye  is 
hyperopic.  We  must  tlicn  try  successively  strong- 
er convex  lenses  until  we  find  the  one  which  brings 
the  point  of  reversal  at  one  meter,  and  the  strengtli 
of  til  is  lens  in  excess  of  1  D.  is  the  measure  of 
divergence  of  the  emerging  rays,  i.  e.,  of  the  hy- 


118  KEFRACTIOX 

peropia  of  the  eye,  and  the  necessary  correction. 

Example. — The  shadow  is  seen,  to  move  against 
the  mirror.  A  convex  lens  of  1  D.  still  leaves  it 
moving  against.  A  convex  lens  of  2.25  D.  just 
brings  the  point  of  reversal  at  the  observer's  eye, 
so  that  by  moving  slightly  backward  or  forward 
it  is  made  to  move  with  and  against.  Then  -|-2.25 
D.  less  +1  D.,  or  -|-1.25  D.,  is  the  measure  of  the 
hyperopia  and  will  correct  it. 

Direct  Ophthalmoscopy  in  Hypermetropia. 
— Under  the  direct  method  of  ophthalmoscopy  the 
rays  emerging  from  the  patient's  e3^e  are  diver- 
gent, and  the  normal  unaccommodated  eye  of  the 
observer  cannot  focus  them  upon  the  retina,  and 
therefore  cannot  get  a  clear  view  of  the  patient's 
fundus.  If  a  convex  lens  wheeled  in  front  of  the 
sight  hole  improves  the  image  we  know  that  the 
emerging  rays  are  divergent,  i.  e.,  the  patient  is 
hyperopic.  and  the  strongest  convex  lens  which 
enables  the  observer  to  get  a  clear  view  of  the 
patient's  fundus,  i.  e.,  which  renders  the  diver- 
gent emerging  rays  equivalent  to  parallel  rays,  is 
of  course  the  measure  of  their  divergence,  i.  e., 
of  the  hyperopia  of  the  eye,  and  its  correction. 

Example. — Under  direct  ophthalmoscopy,  with 
no  lens  at  all,  the  observer  cannot  ^ee  the  fundus 
of  the  patient's  eye.  A  weak  convex  lens  improves 
it,  and  a  -|-2  D.  is  the  strongest  lens  with 
which  a  clear  view  is  obtainable.  Then  the  pa- 
tient has  2  D.  of  hyperopia  and  a  +2  D.  lens  is 
his  proper  correction. 


{■()i!i;i;(  rrox  OF  iiyimikmki  koima  11^ 

Indirect  OriiTiiALMOscoPY  in  Hypermetro- 

PiA. — ^IJnder  tlic  indirect  method,  the  rays  from  the 
patient's  eye,  being  divergent,  will  strike  the  ob- 
jective lens  at  a  more  and  more  divergent  angle 
the  further  away  the  objective  is  held  from  the 
patient's  eye,  and  will  therefore  be  brought  to 
focus  further  and  further  beyond  the  focal  length 
of  the  objective  lens,  making  a  smaller  and  smaller 
image. 

We  now  mount  before  the  patient's  eye  suc- 
cessively stronger  convex  lenses  until  we  find  the 
strongest  one  which  renders  the  image  the  same 
size,  no  matter  at  what  distance  the  objective  is 
held;  that  is,  the  size  of  the  image  neither  in- 
creases nor  diminishes  as  we  withdraw  the  ob- 
jective. This  means  that  the  convex  lense  in 
question  has  rendered  the  emerging  rays  just  par- 
allel, and  is  therefore  the  measure  of  their  di- 
vergence, i.  e.,  of  the  hyperopia  of  the  eye,  and 
will  correct  it. 

Example. — Under  indirect  ()})litlialmoscopy  the 
image  of  the  lens  is  found  to  diminish  in  size 
as  we  withdraw  the  objective.  Under  successive- 
Iv  stronger  convex  lenses  mounted  before  the  eye 
tlie  diminishing  becomes  less  and  less,  until  with  a 
convex  lens  of  2  D.  the  image  remains  the  same 
size  as  we  withdraw  the  objective.  Then  the  eye 
has  2  D.  of  hyperopia,  and  a  +2  TX  lens  is  its 
proper  correction. 


CHAPTER  X. 

COKRECTIOX  OF  MYOPIA. 

Myopia^  as  already  explained,  is  that  condition 
of  the  eye  in  which  parallel  rays  are  brought  to 
focus  in  front  of  the  retina.  In  other  words,  the 
antero-posterior   diameter   of   tlie   eye-l)all   is   too 


A 
B- 
A 
B 


Showing  the  effect  of  chromatic  refraction  in  my- 
opia. The  blue  rays  (light  lines  AA)  have  focused 
and  crossed  by  the  time  they  reach  the  retina,  while 
the  red  rays  (heavy  lines  BB)  have  just  come  to  fo- 
cus,   making   a  red   centre   surrounded    by   blue. 


long  in  proportion  to  tlie  refracting  power  of  the 
eye,  and  the  principal  focal  point  is  situated  in 
front  of  tlie  retina. 

Chho:\[atic  Test. — In  the  niyojur,  or  long  eye, 
the  violet  ra3's,  being  refracted  more  quickly  than 
the  red  rays,  have  already  come  to  a  focus  anil 
have  crossed  by  tlie  time  tlicy  reach  the  retina; 
the  red  rays,  Ixung  U'ss  aiVected  by  refraction,  liave 
either  not  yet  met  or  have  just  come  to  a  focus  of 
the  retina.  Hence  llie  apjX'arance  of  tlie  flame  is 
that  of  a  violet  (or  blue)  margin  annind  a  red 
center.  This  appearance  is  diagnostic  of  (lie  my- 
opic eye. 


122  HKH{A(  TIOX 

Distant  Type  Test  for  Myopia. — The  myopic 
eye  is  unable  to  distinguish  the  letters  of  N^o.  6 
test  type  at  6  meters,  because  the  parallel  rays 
which  proceed  from  those  letters  are  focused  be- 
fore they  reach  the  retina,  and  as  the  patient's 
eye  is  already  in  a  state  of  rest,  i.  e.,  it  has  as- 
sumed the  least  convex  form  of  which  it  is  capable, 
it  has  no  further  means  of  adjusting  itself  to  the 
too-far-forward  focus. 

It  is  true  that  an  astigmatic  eye  is  also  unable 
to  distinguish  No.  6  type  at  6  meters,  because  of 
its  inability  to  focus  the  parallel  rays  which  enter 
it  along  its  defective  meridian,  and  it  is  some- 
what difficult  to  differentiate  at  once  between  a 
simply  myopic  and  an  astigmatic  eye.  In  astig- 
matism, however,  the  ability  to  decipher  the  type 
is  as  a  rule  not  so  complete  as  in  myopia;  the 
patient  is  generally  able  to  read  the  type,  with  an 
effort.  And  if,  in  addition,  the  spokes  of  the  as- 
tigmatic chart  appear  all  black  alike  to  him,  then 
it  is  pretty  safe  to  proceed  as  if  the  error  were 
simple  myopia.  But  the  tests  for  astigmatism, 
hereafter  to  be  described,  should  in  every  case  be 
fully  applied. 

Lenses  foh  ]\Iyopia. — A  convex  lens  held  be- 
fore the  myopic  eye  of  course  renders  tlie  vision 
still  less  distinct,  because  it  still  further  hastens 
tbe  focusing  of  the  parallel  rays,  and  therefore 
makes  the  circle  of  diffusion  produced  by  the 
crossed  rays  on  the  retina  still  larger  in  area.  A 
weak  concave  lens,  on  the  other  hand,  improves  the 


rORHK(   I  ION     Ol      MVoriA  123 

vision,  because  it  delays  (lie  focussing  of  the 
rays,  thus  lessening  the  area  of  the  circle  of  dif- 
fusion. We  then  try  successively  stronger  con- 
cave lenses,  until  we  find  the  weakest  concave 
lens  with  which  the  patient  can  clearly  decipher 
No.  6  type  at  6  meters.  This  is  the  lens  which 
renders  the  rays,  as  they  strike  the  myopic  eye, 
equivalent  to  i)ai'allel  rays  entering  a  normal  eye, 
and  is  therefore  the  measure  of  tlic  myopia  of 
the  eye,  and  will  be  the  proper  correction. 

Eetinoscopy  in  Myopia. — Under  retinoscopy 
the  rays  emerging  from  the  patient's  eye  are  con- 
vergent. If  the  degree  of  myopia  is  a  high  one,  i. 
e.,  if  the  emerging  rays  are  very  convergent,  they 
will  have  met  and  crossed  before  they  reach  the 
observer's  eye  at  one  meter  distance;  but  if  the 
degree  of  myopia  is  a  low  one,  i.  e.,  if  the  emerg- 
ing rays  are  only  slightly  convergent,  tliey  will 
not  have  met  by  the  time  they  reach  the  observ- 
ers eye,  but  will  meet  before  reaching  infinity  ((> 
meters). 

In  the  latter  case,  i.  e.,  in  low  degrees  of  myo- 
pia, the  shadow  will  l)e  seen  to  move  against  the 
mirror,  the  same  as  in  cnimetropia  and  hyperopia, 
but  it  will  fake  a  convex  lens  of  less  dioptric 
strength  to  get  the  ])()iiit  of  reversal  a  I  1  meter 
than  is  required  to  bring  parallel  rays  to  a  focus 
at  that  point;  it  will,  in  fnct,  require  a  convex- 
lens  just  as  much  less  than  1  I),  as  the  rays  are 
convergent,  hence  the  amount  that  the  reversing 


124  KEFR ACTION 

lens  is  in  default  of  1  D.  is  the  measure  of  the 
myopia  of  the  eve. 

In  high  degrees  of  myopia,  the  shadow  will  at 
once  be  seen  to  move  with  the  mirror,  because 
the  emerging  rays  have  alreadv  crossed  by  the 
time  they  reach  the  observer's  eye  at  one  meter. 
It  will  now  take  a  concave  lens  to  bring  the  point 
of  reversal  at  this  distance;  that  is,  the  focussing 
of  the  rays  must  be  hindered,  and  brought  for- 
ward to  the  1  meter  point.  We  therefore  find  the 
vreakest  concave  lenst  which  just  reverses  the 
shadow.  [N'ow  we  know  that  if  the  emerging  ravs 
had  just  met  at  1  meter  the  eye  would  be  just 
1  D.  myopic.  The  concave  lens,  therefore,  which 
delays  the  focussing  so  that  they  meet  at  just  1 
meter  is  the  measure  of  the  convergence  of  the 
emerging  rays  in  excess  of  1  D.  Hence  the 
strength  of  this  concave  lens,  added  to  1  D.,  is 
the  total  measure  of  the  myopia  of  the  eye,  and 
will  be  its  proper  correction. 

Examples. — Shadow  is  seen  to  move  against 
the  mirror.  A  0.25  lens  just  reverses  the  shadow 
movement.  Then  the  eye  is  myopic  to  the  degree 
that  0.25  D.  is  less  than  1  D.,  i.  e.,  it  is  myopic 
0.75  D.,  and  a  —0.75  D.  lens  will  correct  the 
myopia. 

Shadow  is  seen  at  once  to  move  with  the  mir- 
ror. A  — 1.00  D.  lens  just  reverses  the  move- 
ment. Then  the  eye  is  myopic  to  the  degree  that 
1  D.  is  in  excess  of  1  D.,  i.  e.,  it  is  myopic  2  D , 
and  a  — 2  D.  lens  will  correct  ii 


COKRKCTION    OF    MYOl'lA  125 

Direct  Ophthalmoscopy  in  Myopia. — Under 

tlie  direct  nietliod  of  ophthalmoscopy  the  rays 
emerging  from  the  patient's  eye  are  convergent, 
and  the  normal  unaccommodated  eye  of  the  ob- 
server cannot  focus  them  upon  his  •  etina,  hence 
cannot  get  a  clear  view  of  the  patient's  fundus.  A 
convex  lens  wheeled  before  the  sight  hole  makes 
matters  worse,  as  it  converges  the  rays  still  more. 
We  therefore  try  concave  lenses,  and  the  weakest 
concave  lens  which  enables  the  observer  to  get  a 
clear  view  of  the  patient's  fundus,  i.  e.,  which 
renders  the  convergent  emerging  rays  equivalent 
to  parallel  rays,  is  of  course  the  measure  of  their 
convergence,  i.  e.,  of  the  myopia  of  the  eye,  and  its 
correction. 

Example. — Under  direct  ophthalmoscopy,  with 
no  lens  at  all,  the  observer  cannot  see  the  details 
of  the  patient/s  fundus.  A  weak  convex  lens  makes 
is  still  more  indistinct,  but  a  weak  concave  lens 
improves  it,  Mid  — 1.50  D.  is  the  weakest  con- 
cave lens  with  which  a  clear  view  is  obtainable. 
Then  the  patient  has  1.50  D.  of  myopia,  and  a 
— 1.50  D.  lens  is  his  proper  correction. 

Indirect  Ophthalmoscopy  in  Myopia. — Un- 
der the  indirect  method,  the  rays  from  the  pa- 
tient's eye,  being  convergent,  will  strike  the  ob- 
jective lens  at  a  more  and  more  convergent  angle 
the  further  away  the  objective  is  held  from  the  pa- 
tient's eye,  and  will  therefore  be  brought  to  focus 
nearer  and  nearer  within  the  focal  length  of  the 
objective  lens,  making  a  larger  and  larger  image. 


12G  KEl'KACTION 

We  now  mount  before  the  patient's  eye  succes- 
sively stronger  concave  lenses  until  we  find  the 
weakest  one  which  renders  the  image  the  same 
size  at  whatever  distance  the  objective  is  held;  that 
is.  the  size  of  the  image  neither  decreases  nor  in- 
creases as  we  withdraw  the  objective.  This  means 
that  the  concave  lens  in  question  has  rendered  the 
emerging  rays,  just  parallel;  it  is  therefore  the 
measure  of  their  convergence,  i.  e.,  of  the  myopia 
of  the  eye,  and  will  correct  it. 

Example. — Under  indirect  ophthalmoscopy  the 
image  of  the  disc  is  found  to  increase  in  size  as 
we  withdraw  the  objective.  Under  successively 
stronger  colicave  lenses  the  enlarging  is  seen  to 
be  less  and  less,  until  with  a  concave  lens  of  1.50 
1).  the  image  remains  the  same  size  as  the  ob- 
jective is  withdrav/n.  Then  the  eye  has  1.50  D. 
of  myopia,  and  a  — 1.50  D.  lens  is  its  proper  cor- 
rection. 


CHAPTER  XI. 
CORKECTION  OF  ASTIGMATISM. 

Astigmatism  is  by  far  the  most  troublesome  to 
estimate  and  correct  of  all  the  commoner  refrac- 
tive errors  of  the  eye.  It  has  already  been  de- 
scribed as  that  condition  of  tlie  eye  in  which  the 
curvature  of  the  cornea  is  not  the  same  in  all 
of  its  meridians,  the  result  being  that  while  the 
rays  which  enter  the  normal  meridians  are  prop- 
erly focussed  on  the  retina,  those  which  enter 
tlirough  the  defective  meridians  form  a  line  of 
unfocussed  rays,  either  in  front  of  or  behind  the 
retina,  and  a  diffused  indistinct  image  is  seen. 

Chief  Meridians. — As  we  have  already  seen, 
the  meridians  of  greatest  and  lease  refraction  are 
practically  always  at  right  angles  to  each  other, 
and  are  called  the  chief  meridians. 

Prismatic  Test. — In  order  to  carry  out  the 
chromatic  test  for  astigmatism  it  is  necessary  to 
place  the  white  flame  behind  a  screen  having  a 
small  circular  hole  about  G  mm.  in  diameter  im- 
mediately in  front  of  the  light,,  so  that  not  too 
many  rays  may  enter  the  eye.  The  chromatic 
lens  is  then  mounted  before  the  eye.  Of  the  rays 
which  pass  through-  the  meridian  of  greatest  re- 
fraction the  bhu}  rays  come  to  focus  first,  while 
of  those  which  enter  the  meridian  of  least  re- 
fraction the  red  rays  come  to  focus  quickest,  and 
as  these  two  meridians  are  at  right  augles  to  each 
other  the  two  colors  appear  drawn  out  at  right 


128  KEFKACTION 

angles  corresponding  to  the  chief  meridians.  This 
test  only  applies  in  high  degrees  of  astigmatism. 

Test  Type  in  Astigmatism. — It  is  manifest 
that  the  rays  of  light  proceeding  from  the  distant 
type  cannot  be  correctly  focussed  upon  the  retina 
by  both  of  the  chief  meridians  at  once.  If  one  is 
adapted  to  focus  the  ra3^s,  the  other  is  not.  Hence 
the  distant  type  cannot  be  seen  with  distinctness 
by  an!  astigmatic  eye.  This  inability  of  the  eye 
to  see  the  distant  type  clearly  is  not  in  itself  a 
proof  of  astigmatism^  for  we  have  seen  that  it 
pertains  to  the  myopic  eye  also.  But  if  the  dis- 
tant type  can  be  read  clearly  it  is  sufficient  evi- 
dence that  there  is  no  astigmatism. 

Wheel  Test  ix  Astigmatism. — We  have  al- 
ready seen  that  the  simplest  method  of  detecting 
the  deficiency  of  one  of  the  meridians  of  the  eye 
is  to  test  the  ability  of  the  eye  to  discern  a  series 
of  black  lines  arranged  to  correspond  with  the 
ocular  meridians.  We  therefore  instruct  the  pa- 
tient to  look  at  the  wheel  chart  and  pick  out  the 
spokes  that  appear  to  him  the  faintest.  These 
spokes  represent  the  meridian  at  right  angles  to 
the  defective  meridian  of  the  eye.  JN'ow  it  is  evi- 
dent that  whatever  spherical  lens  will  focus  the 
rays  in  this  meridian,  i.  e.,  bring  out  the  faint 
lines  clear  and  l)lack,  will  be  the  measure  of  the 
character  and  degree  of  defect  in  the  meridian. 

We  therefore  instruct  the  patient  to  look  at  the 
wheel  and  pick  out  the  spokes  that  appear  the 
faintest  to  him.    Begin  wit1i  a  weak  convex  spher- 


CORRECTION  OF  ASTIGMATISM  129 

ical  lens  and  see  if  it  improves  these  faint  lines. 
If  it  does,  mount  a  strong  convex  spherical  lens_, 
as  in  the  nicisiircmcnt  oL'  liypormetropia,  and 
gradually  nMlucr  it  with  coiujne  lenses,  until  the 
spokes  of  the  wheel  which  were  faintest  to  the 
naked  eye  are  clear  and  distinct.  The  convex 
splierical  correction  necessary  to  accomplish  this 
is  the  measure  of  the  convex  cylinder  which,  with 
its  axis  at  right  angles  to  the  defective  meridian, 
w  ill  correct  tlie  astigmatism. 

If  a  convex  lens  does  not  improve  the  weakest 
lines,  but  ])lurs  them,  then  try  a  concave  spher- 
ical lens  until  the  weakest  is  found  which  brings 
the  faintest  lines  out  clear  and  black.  This  lens 
is  the  measure  of  the  concave  cylinder  which, 
with  its  axis  perpendicular  to  tlie  defective  merid- 
ian, will  correct  the  astigmatism. 

As  this  method  is  applicable  only  to  simple 
astigmatism,  i.  e.,  astigmatism  in  which  only  one 
meridian  is  defective,  it  may  be  impossible  to  get 
any  results  from  it,  in  which  case  the  observer 
must  proceed  to  other  methods. 

The  Stenopaic  Slit  ix  AsTiGMATis:\r. — In 
the  trial  case  will  be  found  on  opaque  disc  wn'th  n 
narrow  slit  in  it.  This  is  called  the  stenopaic  slit. 
The  effect  of  this  disc,  when  mounted  before  the 
eye,  is  to  cut  out  all  the  rays  of  light  except  those 
which  enter  along  the  line  of  the  slit,  and  liciice, 
])y  turning  the  disc  so  as  to  make  the  slit  coincidi' 
with  the  various  meridians  of  the  eye,  all  I'ays 
will  b(^  exchided  exce])ting  those  which  cnt<'r  along 


130 


REFRACTION 


the  meridian  coinciding  with  tlie  direction  of  th.o 
slit. 

Now  if  the  eye  he  equally  convex  in  all  of  its 
meridians^  vision  will  he  equally  clear  at  what- 
ever angle  the  stenopaic  slit  1  e  turned,  because 
any  meridian  of  the  normal  eye  will  focus  tlie 


The   Stenopaic   Slit. 

rays  ujoon  the  retina  with  equal  correctness.  If, 
however,  the  meridians  of  the  eye  are  not  equally 
convex,  then  it  is  manifest  that  when  the  slit  is 
turned  to  coincide  respectively  with  the  two  chief 
meridians  the  vision  will  be  correspondingly  most 
and  least  distinct.  It  must  always  be  borne  in 
mind,  however,  that  the  meridian  whose  condition 
is  made  manifest  by  the  slit  is  the  meridian  at 
right  angles  to  the  direction  of  the  slit.  Con- 
versely, it  must  be  remembered  that,  in  estimating 
and    correcting   astigmatism    with    the    stenopaic 


CORRECTION    OF    ASTIGMATISM  131 

i^\'\[,  the  axis  of  the  correcting  cylinder  should  be 
at  i-iglit  angles  to  the  defective  meridian,  i.  e., 
parallel  to  the  slit. 

^lounting  this  disc,  then,  on  the  trial  frame,  we 
turn  it  around  until  the  angle  of  the  slit  is  found 
which  gives  the  patient  the  best  possible  vision 
of  the  distant  type.  If  this  vision  is  6/6,  the 
refraction  of  the  patient^s  best  meridian  is  normal, 
and  if  he  has  astigmatism  at  all  it  is  a  case  of 
simple  astigmatism. 

Now  turn  the  slit  at  right  angles  to  its  best 
position.  This  gives  the  eye  its  worst  vision.  If 
at  this  worst  angle  the  vision  is  still  6/6,  the  eye 
is  normal  in  both  meridians;  no  astigmatism  ex- 
ists. If  not,  we  find  the  spherical  lens  (either 
convex  or  concave)  which  enables  the  patient  to 
read  No.  G  type,  and  this  is  the  measure  of  the 
astigmatism  of  the  meridian  at  right  angles  to 
the  slit.  A  cylindrical  lens,  of  the  same  curva- 
ture and  strength,  with  its  axis  at  right  angles 
to  the  defective  meridian,  i.  e.,  with  its  axis  par- 
allel to  the  slit,  will  correct  the  astigmatism. 

Simple  Astigmatism. — If,  with  tlic  slit  at  its 
best  angle,  vision  is  6/6,  then  only  one  meridian 
of  the  eye  is  astigmatic  (simple  astigmatism). 
If,  with  the  slit  at  its  worst  angle,  a  convex 
spherical  lens  is  required  to  make  vision  6/6,  this 
means  thatf  the  meridian  at  right  angles  to  the 
slit  is  hypermetropic.  The  strength  of  the  lens 
iMii ployed  is  tlie  measure  of  the  hypcrmetropia  of 
the   defective  mcriflian   and   of  the  convex   cvlin- 


132  REFRACTION 

der  which,  with  its  axis  at  right  angles  to  the 
defective  meridian  (parallel  to  the  slit),  will  cor- 
rect it.   - 

If,  on  the  other  hand,  with  the  slit  at  its  worst 
angle,  a  concave  spherical  lens  is  required  to  make 
vision  normal,  then  the  meridian  at  right  angles 
to  the  slit  is  myopic.  The  strength  of  the  concave 
lens  employed  is  the  measure  of  the  myopia  of  the 
defective  meridian,  and  of  the  concave  cylinder 
which,  with  its  axis  at  right  angles  to  the  defective 
meridian  (parallel  to  the  slit)  will  correct  it. 

CoMPOLXD  AstiCt.viatism. — If  a  lens  was  needed 
to  make  the  vision  6/6,  hoth  with  the  slit  at  its 
best  and  at  its  worst  angle,  then  both  of  the 
meridians  of  the  eye  are  astigmatic  (compound 
astigmatism).  If  the  lenses  requiredl  to  correct 
the  best  and  worst  meridian  are  of  the  same  curva- 
ture (i.  e.,  both  plus  or  both  minus)  the  astigma- 
tism is  a  compound  hypermetropic  or  compound 
myopic  astigmatism,  as  the  case  may  be. 

In  this  case  we  ascertain,  by  means  of  the 
stenopaic  slit,  the  degree  of  error  in  each  of  the 
two  chief  meridians,  one  of  which  will  of  course 
be  greater  tlian  the  other,  for  if  tlie  two  chief 
meridians  were  equally  defective  the  case  would 
not  be  one  of  astigmatism  at  all,  but  of  simple 
jiypermetropia  or  myopia.  In  other  words,  we  as- 
certain the  respective  strengths  of  the  lenses,  con- 
vex or  concave,  which  raise  vision  to  normal  with 
the  slit  at  the  ])ost  and  worst  angles  respectively. 

Now   it   is   evident   that   if  the   worst  ineridian 


CORRECTION   OF   ASTIGMATISM  133 

be  corrected  to  the  extent  of  the  difference  be- 
tween it  and  the  best  meridian,  tlicn  both  merid- 
ians will  be  equally  defective  to  the  extent  of  the 
best  meridian,  and  we  shall  have  reduced  the  case 
to  one  of  simple  hypermetropia  or  myopia,  as  the 
case  may  be,  equal  to  the  defect  of  the  best  merid- 
ian. 

Having  found  the  respective  degrees  of  error 
of  the  two  chief  meridians,  therefore,  the  proper 
way  to  correct  it  would  be  to  prescribe  a  cylin- 
drical lens,  equal  to  the  difference  between  the 
two  meridians,  with  its  axis  at  right  angles  to 
the  worst  meridian — this  makes  the  defects  of  the 
two  meridians  equal,  and  renders  the  case  one  of 
simi)le  hyperopia  or  myopia — and  then  prescribe 
a  spherical  lens  equal  in  strength  to  the  error  of 
the  best  meridian.  This  corrects  the  remaining 
hyperopia  or  myopia,  and  renders  the  eye  normal. 

Example. — The  slit  at  90°  (vertical)  gives  the 
best  possible  vision,  but  this  vision  is  not  Q/(d, 
and  requires  a  -|-1  D.  spherical  lens  to  make  it 
normal.  This  means  that  the  horizontal  meridian 
of  the  eye  is  hyperopic  1  D.  At  180°,  the  worst 
possible  angle,  it  requires  a  +2.50  D.  spherical 
lens  to  make  vision  6/0.  This  meanS;  that  the 
vertical  meridian  is  hyperopic  2.50  D.  Here  the 
difference  between  the  two  meridians  is  -["1.50  D. 
Hence  a  -f  1.50  D.  cylinder,  with  its  axis  at  right 
angles  to  the  worst  meridian,  i.  e.,  axis  at  180°, 
will  neutralize  the  astigmatism,  makino-  the  two 
f'hief  rneridiaps  both  hyperopic  1  D.,  and  a  spher^ 


134  REFRACTION 

ical  lens  of  +1.00  D.  will  then  correct  this  re- 
maining hyperopia. 

With  both  meridians  myopic,  the  result  woukl 
work  out  precisely  the  same  in  concave  lenses. 

AlixED  Astigmatism. — This  is  the  type  of  astig- 
matism in  which  the  two  chief  meridians  are  both 
defective,  but  of  different  curvature,  one  being 
liyperopic  and  the  other  myopic.  In  this  case  we 
ascertain  by  means  of  the  stenopaic  slit  the  char- 
acter and  degree  of  the  defect  in  each  meridian. 
In  other  words  we  find  the  respective  strengths  of 
the  lenses,  convex  in  one  meridian  and  concave 
in  the  othei%  which  raise  vision  to  6/6  with  the 
slit  at  its  best  and  worst  angle  respectively. 

Now  it  is  manifest  that  if  either  of  these  merid- 
ians be  corrected  to  the  extent  of  the  sum  of  the 
errors  of  the  two  meridians,  it  will  be  over-cor- 
rected to  the  extent  that  the  other  is  defective,  and 
the  case  will  tlien  be  reduced  to  one  of  simple 
hyperopia  or  nw opia ;  as  the  case  may  be,  equal 
to  the  defect  of  the  uncorrected  meridian. 

Having  found,  therefore,  the  respective  degrees 
of  error  of  the  two  chief  meridians,  the  proper  way 
to  correct  them  is  to  prescribe  a  cylindrical  lens, 
equal  in  strength  to  the  sum  of  tlie  two  errors, 
and  of  appropriate  correcting  curvature,  with  its 
axis  at  right  angles  to  the  meridian  requiring  that 
curvature  lens — this  over-corrects  the  error  of  this 
meridian  nnd  renders  it  defective  to  the  same  de- 
gree and   in   the  same  direction   as  the   opposite 


CORRECTION   OB^   ASTIGMATISM  135 

meridian,  rendering  the  case  one  of  simple  hyper- 
opia or  myopia — and  then  prescribe  a  spherical 
lens  equal  in  strength  and  curvature  to  the  error 
which  now  exists  similarly  in  both  meridians. 
This  corrects  the  remaining  hyperopia  or  myopia, 
51  nd  renders  the  eye  normal. 

J  n  these  cases  it  is  customary  to  over-correct 
with  the  cylinder  the  meridian  of  least  convexity, 
i.  e.,  the  one  which  requires  the  plus  lens.  How- 
ever, this  will  not  be  found  practicable  in  all 
cases,  as  instances  will  be  found  wherein,  for  some 
reason  or  other,  this  form  of  correction  does  not 
give  as  good  vision  as  the  reverse. 

Example. — The  slit  at  90°  (vertical)  gives  best 
possible  vision,  but  this  vision  is  not  Q/Q,  and  re- 
quires a  +1  D.  spherical  lens  to  make  it  normal. 
This  means  that  the  horizontal  meridian  is  hyper- 
opic  1  D.  At  180°  (horizontal),  the  worst  pos- 
sible angle,  it  requires  a  — 1.50  D.  spherical  lens 
to  make  vision  Q/Q.  This  means  that  the  ver- 
tical meridian  is  myopic  1.50  D.  Here  the  sum 
of  the  two  errors  is  2.50  D.,  and  the  horizontal 
meridian  is  the  meridian  of  least  convexity.  Hence 
a  cylinder  of  -f  2.50  D.,  with  its  axis  at  right 
angles  to  the  horizontal  meridian,  i.  c.,  axis  at  90°, 
will  over-correct  that  meridian  so  that  it  becomes 
myopic  1.50  D.,  the  same  as  the  vertical  meridian. 
The  two  meridians  are  now  both  myopic  1.50  D.. 
a  case  of  simple  myopia,  and  a  spherical  lens  of 
— 1.50  will  correct  this  remaining  myopia. 


136  REFRACTION 

Retinoscopy  in  Astigmatism. 

Straight  and  Oblique  Edges. — In  an  astig- 
matic eye  the  image  of  the  light  which  is  made 
upon  the  retina  is  oval,  with  its  greatest  and  least 
diameters  corresponding  to  the  least  and  great- 
est curvature  of  the  cornea.  This  oval  may  have 
its  edges  vertical,  horizontal,  or  oblique,  accord- 
ing to  the  nature  of  the  astigmatism. 

When  the  astigmatism  is  either  directly  with 
or  directly  against  the  rule,  i.  e.,  horizontal  or 
vertical,  the  edges  of  the  shadow  are  vertical  and 
horizontal,  and  move  horizontally  or  vertically 
across  the  pupil,  giving  the  effect  of  a  straight 
shadow  as  in  hyperopia  or  myopia. 

When  the  astigmatism  is  oblique  the  edges  of 
the  shadow  are  oblique,  and  move  obliquely  across 
the  pupillary  field  no  matter  how  the  mirror  is 
rotated.  Hence,  while  oblique  shadows,  when 
found,  are  diagnostic  of  astigmatism,  straight  sha- 
dows do  not  by  any  means  exclude  astigmatism. 

Testing  Both  Meridians. — The  only  safe 
method  to  pursue  is  to  make  a  routine  practice,  in 
cases  of  vertical  or  horizontal  shadows,  of  testing 
both  meridians  with  the  retinoscope. 

Example. — We  find  the  shadow  vertical,  and 
moving  against  tlie  mirror.  We  first  test  tlie 
vertical  meridians,  rotating  the  mirror  liorizontal- 
ly,  and  find  the  strongest  convex  lens  which  causes 
the  shadow  to  move  with  the  mirror.  Suppose  this 
is  found  to  be  +2  D.  We  then  test  the  horizontal 
meridian,  rotating  the  mirror  vertically,  and  find 


CORRECTION  OF   ASTIGMATISM 


137 


the  strongest  convex  lens  which  makes  the  sliadow 
move  with  the  mirror  is  -\-2  D.  We  know  that 
the  meridians  are  the  same  and  the  case  is  one  of 
simple  hypermetropia. 

Suppose^  however^  in  testing  the  horizontal 
meridian  we  had  found  that  -f-i  I^-  ^^'^^^  the  strong- 
est convex  lens  making  the  shadow  move  witli  the 
mirror.  Then  we  have  a  compound  hypermetropic 
astigmatism,  2  D.  in  the  vertical  meridian  and  4 


Illustrating  the  way  the  mirror  must  be  held  and 
rotated  when  the  shadow  is  oblique,  viz.,  perpendicu- 
lar  to  the  edge  of  the  shadow. 

D.  in  the  horizontal,  needing  for  its  correction  -|-2 
1).  cylinder,  axis  horizontal,  and  -\-2  D.  spherical 
(i.  c.,  tlie  difference  between  the  two- meridians). 
WJiere  the  shadow  is  oblique,  we  know  at  once 
that  astigmatism  exists,  and  proceed  to  eslini;iit' 
it  as  above,  only  in  this  case  wc  rotate  the  inin-or 
at  right  angles  to  the  edge  of  the  shadow  and  then 
parallel  to  it.  and  the  axis  of  the  correcting  cylin- 
der is  placed  accordiiiLrly.  Thus,  if  the  edge  of 
the  shadow  is  tilted  at  'iU^  tlie  cylinder  is  pre- 
scribed with  its  a.xis  at  110°,  which  is  precisely 
at  right  angles  thereto. 


138  REFRACTION 

Astigmatic  Band. — In  cases  of  astigmatisiii  in 
which  one  of  the  meridians  has  received  its  spher- 
ical correction  the  reflection  of  the  retinoscope  is 
usually  seen  as  a  band  of  light  lying  across  the 
pujiil^  whose  axis  subtends  the  uncorrected  merid- 
ian. Hence  in  refracting  the  horizontal  meridian^ 
if,  as  soon  as  the  movement  of  the  shadow  in  this 
meridian  is  neutralized,  a  band  of  light  is  seen 
\\ing  horizontally  across  the  pupil,  the  observer 
knows  that  the  eye  is  astigmatic,  and  that  the  ver- 
tical meridian  is  the  astigmatic  one,  and  should 
proceed  to  refract  that  meridian. 

In  cases  where  the  difference  between  the  two 
meridians  is  very  slight  this  band  does  not  of 
course  appear  until  the  correction  of  the  merid- 
ian first  tested  is  very  near  completion,  and  it  is 
then  seen  very  faintly.  For  this  reason,  among 
others,  it  is  advisable,  as  stated  in  a  previous  lec- 
ture, to  reduce  the  area  of  the  illuminating  disc 
when  one  gets  very  near  to  the  neutralizing  point. 

Correction, — Having  ascertained,  by  neutral- 
izing the  movements  of  the  shadow  in  the  two 
chief  meridians,  the  nature  and  degree  of  error  in 
each,  the  correction  is  then  made  in  the  same  man- 
ner as  already  des(]'il)(Ml  under  the  stenopaic  slit, 
namely  by  a  combination  of  cylinder  and  sphere 
arranged  as  already  explained. 

Betiiioscopij  is  a  verv  useful  and  rapid  means 
of  estimating  and  correcting  astigmatism  with 
experienced  refractionists,  who  are  usually  able 
to  do  it  by  this  method  with  great  accuracy.    But 


CORRECTION   OF   ASTIGMATISM  1^0 

beginners  will  find  it  somewhat  diiiicuit,  and  no 
one  should  ever  rely  on  it  without  confirmation  by 
some  other  metliod. 

Ophthalmoscopy  in  Astigmatism. 

Under  ophthalmoscopy,  direct  method,  the  plan 
ite  to  lind  the  spherical  lens,  convex  and  concave, 
which  affords  a  clear  view  of  the  principal  me- 
ridians, testing  one  at  a  time. 

By  the  indirect  method  the  disc  is  oval  instead 
of  circular,  and  increases  or  decreases  in  one  di- 
rection more  than  another  on  withdrawing  the 
ohjective.  The  spherical  lens,  convex  and  con- 
cave, which  corrects  this  is  the  measure  of  the 
astigmatism  in  each  meridian,  which  must  then 
be  corrected  as  already  described. 

This  method  of  estimating  astigmatism,  how- 
ever, is  so  diflicult  and  untrustworthy  that  it  is  of 
value  only  from  a  scientific  or  confirmatory  point 
of  view  and  i<  not  recommended  for  any  other 
purpose. 

The  stenopaic  slit  will  generally  be  found,  in 
ordinary  cases,  the  most  serviceable  and  prac- 
ticable method  of  working  out  astigmatism,  and 
the  author  strongly  commends  it  to  hi?  readers  as 
superior  to  all  others.  All  findings  should.  Imw- 
ever.  be  confiinu'd  liv  all  tlie  means  at  one's  com- 
mand. 


CHAPTER  Xn. 
PRACTICAL  INSTRUCTIONS. 

General  Procedure. 

Seat  the  patient  in  a  good  light,  as  nearly  as 
possible  six  meters  from  the  distance  type,  using 
the  type-line  marked  No.  6  as  a  standard  of  dis- 
tance vision.  '     • 

(N.  B.  It  is  not  absolutely  essential  that  this 
distance  and  type  number  be  ohserved,  as  any  dis- 
tance beyond  six  meters  renders  the  rays  parallel. 
The  type-line  used  as  a  standard  must  correspond 
with  the  distance/and  if  less  than  6  meters  allow- 
ance must  be  made  for  the  accommodation  that  is 
in  force  in  the  patient's  eye.  But  No.  6  at  6  me- 
ters is  the  generally  used  standard.) 

If  a  good  uniform  natural  light  cannot  be  ob- 
tained, the  test  card  should  he  illuminated  from 
below  with  a  good  artificial  light.  Indeed,  many 
operators  prefer  artificial  light  because  of  its 
greater  uniformity  and  controlability. 

Separate  Tests. — Tost  each  eye  separately, 
shutting  the  other  out  of  vision  meanwhile  by 
means  of  the  opaque  black  disc  found  in  the  trial 
case,  or  by  means  of  a  strong  plus  lens.  Some  op- 
erators prefer  the  latter  because  it  induces  relaxa- 
tion of  the  eye  under  test.  But,  whatever  is  used, 
it  is  important  that  the  untested  eye  be  covered  and 
not  simply  closed  by  the  patient,  because  the  act  of 
cldsinir  ihe  eve  tends  to  interfere  with  aeeonimoda- 


D-60. 


O  E  B  T 


D— 60 


C  T  D  O  E 

D-40 

r  E  D  O  T  c 

I>— 80. 

L  N   F   C   T   D 


D-20 


OPDLEFNC 


OETNFLDBP 


D-10. 


SOBSTKZ.CPF 

Test    Type   CliarL. 
In  the  above  the   "D"   means  distance  and  tlic  number 
is   feet. 


PRACTICAL   INSTRUCTIONS  143 

tion  and  innervation  ui"  ilic  oilier  eye.     Jt  is  cus- 
tomaiy  to  begin  with  tlic  right  eye. 

Make  a  careful  and  systematic  record  of  the 
findings  in  each  test  as  proceeded  with,  designat- 
ing the  right  eye  as  E.  V.  and  the  left  as  L.  V. 

1.  PiN-IIoLE  Test. — Always  begin  with  the 
pin-hole  test.  Instructing  the  patient  to  put  on 
the  trial  spectacle  frame,  mount  before  the  eye 
the  black  disc  with  the  pin-hole  in  the  center 
which  is  in  every  trial  case.  This  has  the  same 
effect  on  the  eye  as  a  cutting  out  light  from  a 
camera,  permitting  rays  to  enter  only  along  its 
central  axis,  and  should  therefore  improve  the 
vision  of  a  healthy  eye. 

If  the  pin-hole  disc  does  not  improve  the 
vision,  there  is  some  physical  trouble  with  the 
eye-ball,  and  an  oculist  must  be  consulted.  If  it 
improves  the  vision,  the  trouble  complained  of 
is  an  error  of  refraction  and  further  tests  should 
be  proceeded  with. 

2.  PiJiSMATic  Test. — If  your  test  case  con- 
tains a  chromatic  lens,  the  prismatic  test  may 
next  be  employed  with  each  eye  separately. 

A  white  light  is  placed  6  meters  from  the  pa- 
tient, and  the  chromatic  lens  mounted  before  his 
eye. 

If  the  patient  sees  a  red  rim  around  a  central 
blue  light,  the  eye  is  hypermetropic. 

If  he  sees  a  blue  rim  around  a  central  red  light, 
the  eye  is  myopic. 

A  screen  is  now  placed  before  the  light,  with 


144  REFRACTION 

a  round  opening  6  mm.  in  diameter  immediately 
in  front  of  the  fiame^  and  if  the  two  colors,  blue 
and  red,  are  seen  as  bars  at  right  angles  to  each 
other,  tlie  eye  is  astigmatic. 

3.  Distance  Type. — The  distance  type  should 
now  be  employed,  each,  eye  being  tested  sep- 
arately. The  patient  is  directed  to  look  at  the 
type  card  and  tell  how  far  down  he  can  read. 

If  at  a  distance  of  20  feet  lie  reads  type  Xn. 
20,  Ills  vision  is  expressed  by  the  fraction  20/20. 
and  is  either  normal  or  hypermetropic, 

Now  mount  a  weak  convex  lens,  say  0.50  or 
0.75  D.,  before  the  eye,  and  if  the  vision  is  not 
blurred  by  it  the  eye  is  hypermetropic,  and  you 
may  proceed  to  measure  the  hypermetropia  by  the 
methods  before  described. 

If  a  convex  lens  does  injure  the  vision,  the  eye 
is  probably  normal  and  needs  only  rest  and  hy- 
gienic treatment.  But  no  eye  should  be  dismissed 
without  other  tests  being  made. 

If  at  a  distance  of  20  feet  he  cannot  read  type 
Xo.  20,  l)ut  can  read,  say  ISTo.  40,  his  vision  is  ex- 
pressed l)y  tlie  fraction  20/40,  and  he  is  either  my- 
opic or  astigmatic — perhaps  both. 

Wheel  Test. — Xow  instruct  the  patient  to  look 
at  the  fan-wheel  which  is  at  the  top  of  the  type 
card,  and  ask  him  if  all  of  the  spokes  of  the  wheel 
look  equally  black  to  him.  If  they  do,  but  he  is 
si  ill  unable  to  read  Xo.  20  lype  at  20  feet,  his 
error  is  simply  myopia,  and  niav  be  measured  and 
corrected  as  di'scrihed  in  a  previous  chapter. 


PRACTICAL    INSTRUCTIONS  145 

If  the  spokes  of  the  wheel  appear  unequally 
black  to  him — one  intensely  black  and  another 
quite  faint — he  has  an  astigmatism,  and  must  be 
dealt  with  as  laid  down  elsewhere. 

Convex  Lens  for  Relaxation. — If  for  any 
reason  neither  atropin  nor  any  otlier  drugs  are 
available,  a  very  satisfactory  relaxation  of  accom- 
modation can  be  obtained  by  mounting  a  convex 
]ens  before  the  eye.  Usually  at  1.50  D.  or  2.00  D. 
is  about  right,  but  the  necessary  strength  varies 
with  the  patient. 

A  sufficiently  strong  diopter  should  be  used  com- 
pletely to  paralyze  accommodation.  When  this 
method  is  used,  the  dioptrism  of  thei  lens  used 
must  of  course  be  added  or  subtracted  from  the 
result  of  the  test,  the  same  as  in  the  case  of 
atropin. 

Spectacles. 
It  is  highly  important  that  the  glasses  them- 
selves be  properly  fitted  to  the  patient's  eyes,  and 
the  refractionist  himself  should  attend  to  this  fea- 
ture of  the  treatment.  Tabulated  instructions 
are  usually  found  on  the  prescription  blanks  is- 
sued by  optical  firms  for  the  proper  measurement 
of  the  frames,  and  these  should  be  fully  and  care- 
fully followed  out  in  prescribing  the  glasses. 

From  an  optical  standpoint  the  most  important 
things  to  be  observed  are: 

1.    That  the  size  of  the  lens  is  sufficient  to  cover 
the   eve.      Optician's   prescription   blanks   usually 


14(>  REFRACTION 

designate  this  by  a  graded  scale  of  sizes^,  repre- 
sented by  0,  00,  000,  etc. 

2.  That  the  center  of  the  lens  coincides  ex- 
actly with  the  visual  axis  of  the  eye.  This  is  in- 
sured by  a  proper  measurement  of  the  width  be- 
tween the  two  pupil  centers.  For  distance  vision 
the  lenses  should  then  be  made  to  stand  perpen- 
dicularly; for  near  vision  they  should  converge  in 
accordance  with  the  visual  axes. 

3.  In  myopia  and  astigmatism  the  lens  should 
not  be  further  than  13  mm.  from  the  eye.  Hy- 
permetropes,  and  especially  presbyopes,  may  gen- 
erally suit  their  own  comfort  and  convenience  in 
this  respect. 

4.  In  astigmatism  it  is  of  course  essential  that 
the  lenses  always  maintain  the  angle  at  which 
they  are  prescribed.  To  insure  this,  it  is  always 
advisable  that  astigmatic  patients  wear  bow-spec- 
tacles. However,  there  are  now  in  the  market  very 
improved  makes  of  eye-glasses  that  provide  for 
this  necessity,  and  these  may  be  worn  in  selected 
cases.  Care  must  be  taken  that  they  do  not  be- 
come bent  and  out  of  shape,  for  this  spoils  the 
tilt  of  the  cylinder  axis  and  defeats  the  purpose 
of  the  lens. 

Latent  Hyperopia. — It  has  already  been  seen 
that  hypermetropes  are  obliged  to  use  some  of 
their  accommodation  for  long  distance  vision. 
Hence  there  is  always  a  certain  amount  of  un- 
detected hyperopia,  called  latent  hypermetropia 
which  is  made  manifest  only  under  atropin,  when 


rRACTICAL   INSTRUCTIONS  147 

the  ciliary  muscle  is  completely  relaxed,  so  that 
the  results  under  atropin  will  be  found  to  be  about 
1  D.  more  than  without  it.  It,  is  advisable  to 
provide  for  about  one-half  of  this  latent  hyper- 
metropia  in  prescribing  the  glass.  Hence  if  the 
tests  have  been  made  under  atropin,  about  0.50 
should  be  deducted  from  the  result  in  prescribing, 
and  if  made  without  atropin  0.50  should  be  add- 
ed, in  prescribing  the  glasses. 

In  hypermetrojyes,  so  long  as  N'o.  6  type  can  be 
read  at  6  meters,  the  glasses  need  only  be  worn 
when  near  work  is  being  done.  If,  however,  even 
distant  vision  is  defective,  they  should  wear  glasses 
to  correct  this.  When  hypermetropia  is  accom- 
panied by  strabismus  (squint)  full  correction 
(latent  and  manifest)  should  be  worn  constantly. 

Myopes  whose  myopia  is  of  low  degree  may  be 
given  eye-glasses  for  use  in  distant  vision,  and 
be  allowed  to  read  and  write  without  any  glasses. 
In  myopia  of  medium,  degree  they  should  wear 
their  full  correction  constantly. 

To  this  general  rule  there  are  two  exceptions: 

A.  Where  the  myopia  is  of  high  degree  the 
concave  glasses  diminish  thq  size  of  the  retinal 
image  so  much  that  the  patient  brings  the  object 
close  to  his  eye  to  make  the  image  larger.  In 
this  case  the  purpose  of  the  glasses  defeats  it- 
self, and  it  is  wiser  to  gradually  accustom  the 
patient  to  his  correction  until  his  acuteness  of 
vision  is  sufficiently  improved  to  stand  full  cor- 
rection. 


148  REFRACTION 

B,  In  cases  of  higli  myojDia  where  the  patient 
has  got  into  the  habit  of  converging  in  excess  of 
his  accommodation,  full  correction,  while  giving 
excellent  distant  vision,  may  cause  him  much  pain 
when  used  for  reading,  and  in  this  event  he  must 
be  given  full  correction  for  distance  and  weaker 
glasses  for  reading,  graduall}-  increasing  the 
strength  of  the  latter  until  he  can  bear  his  full 
correction  for  both  purposes. 

In  these  cases  we  subtract  from  the  full  cor- 
rection the  lens  whose  focal  length  is  the  same 
as  the  distance  at  which  he  desires  to  read.  Thus, 
suppose  his  full  correction  is  — 9  D.,  and  he 
wishes  reading  glasses  for  a  distance  of  33  cm.  A 
3  D.  lens  has  a  focal  length  of  33  cm.  (the  focal 
lengths  of  the  lenses  are  marked  upon  the  trial 
cases),  and  we  therefore  subtract  3  D.  from  — 9 
D.  and  give  him  reading-  glasses  of  — 6  D. 

Hygie>^ic  Treatment. — Myopes  should  be 
carefully  instructed  in  h3'gienic  measures,  even 
after  correction.  They  should  avoid  long  or 
strenuous  convergence,  and  frequently  rest  the  eye 
by  looking  into  the  distance.  They  should  read 
and  write  in  bold  letters,  with  the  paper  at  33 
cm.  at  least  from  the  eye,  and  sedulously  avoid 
the  stooping  posture  when  reading  or  writing, 
which  induces  congestion  of  the  eyeball  and  ag- 
gravates the  myopia. 

^[alignant  Progressive  Myopia. — It  should 
not  be  forgotten  that  myopia  may  be,  and  fre- 
qu(Mitly  is.  a  progressive  and  malignant  condition, 


PRACTICAL   INSTRUCTIONS  149 

and  upon  signs  oi*  such  progression  or  malignancy 
a  competent  oculist  should  be  consulted  without 
delay. 

Astig malic  jinHi'ids  should  wear  their  glasses 
constantly.  If  the  astigmatism  is  associated  with 
high  degrees  of  hypermetropia  or  myopia,  neces- 


Trial    Case. 

sitating  separate  glasses  for  near  and  distant  use, 
the  full  correction  of  the  astigmatism  should  be 
put  into  both  pairs  of  glasses. 

The  astigmatism  is  due,  as  already  seen,  to  the 
irregular  shape  of  the  cornea,  hence  it  does  not 
change  \\ith  accommodation  or  under  any  other 
conditions. 


150  REFRACTION 

Diminishing  Effect  of  Concave  Lenses. — It 
should  not  be  forgotten  that  convex  glasses,  by 
bringing  forward  the  nodal  point,  enlarge  the 
visual  angle  and  so  increase  the  size  of  the  im- 
age, and  concave  glasses,  by  carrying  back  the 
nodal  point,  decrease  the  visual  angle  and  so 
diminish  the  size  of  the  image. 

This  should  be  explained  to  patients,  especially 
those  wearing  concave  glasses,  who  are  apt  to 
complain  that  they  do  not  see  clearly  with  them, 
whereas  the  truth  is  they  do  not  see  as  largely 
with  them. 

Cardinal  Points. 

Always  have  a  good  light,  if  possible  behind  the 
patient,  in  making  the  type  tests. 

Always  test  each  eye  separately,  by  each  method, 
excluding  the  other  eye  meanwhile  by  the  opaque 
disc. 

Always  begin  with  convex  lenses.  They  relax 
the  ciliary  muscle. 

Never  be  satisfied  with  one  kind  of  test.  Con- 
firm it  with  other  methods. 

Always  relax  the  eye,  if  possible,  in  a  patient 
under  30. 

Astigmatism  in  a  young  patient  cannolt  be 
properly  estimated  without  relaxation. 

Look  out  for  ciliary  spasm.  When  the  patient 
begins  to  get  erratic  in  his  replies — seeing  first 
one  thing  and  then  another — stop  testing  and  in- 
sist upon  rest  or  use  a  convex  lens. 

Have    a   system   and   follow   it   out.      Nothing 


PRACTICAL    INSTRUCTIONS  1.31 

weakens  a  patient's  conlidence  and  confuses  the 
refractionist  like  aimless  pottering. 

Having  carefully  attended  to  the  correction  of 
the  refraction,  pa}^  equally  careful  attention  to 
the  mechanical  features  of  the  glasses.  They  are 
of  great  importance,  and  often  make  just  the  dif- 
ference h(  tween  success  and  failure. 


CHAPTER  XIII. 
STKABISMUS  AND  IMBALANCE. 

Strabismus. 

Strabismus    is   that   condition    of    the   eyes   in 
which  the  yellow   spots  are  not  directed  toward 
the  same  point  in  the  object  viewed.     When  the 
'lines  along  which  they  are  directed  diverge  from 
one    another,    the    condition    is    called    divergent 
strabismus ;  when  they  converge  toward  each  other, 
it  is  called  convergent  strabismus.     In  both  cases 
there  is  donblc  vision;  in  the  latter  variety  there 
is  direct  double  vision,  i.  e.,  the  image  seen  by 
each  eye  appears  to  l)e  on  the  same  side  as  the 
eye  perceiving  it.     In  the  former  variety  there  is 
crossed  double  vision,  i.  e.,  the  image  seen  by  each 
eye  appears  to  be  on  the  opposite  side  from  the 
eye  which  perceives  it.     This  is  easily  diagnosed 
by  mounting  a  colored  glass  before  one  eye  and 
obsen^ing  which  image  appears  colored. 
Strabismus  is  real  or  apparent. 
Apparent  strabismus  is  the  effect  of  a  disturb- 
ance  in   the  relationship   between  the  optic   and 
visual  axes,  due  to  hypermetropia  or  myopia.    The 
optic  axis  is  a  line  drawn  through  the  nodal  point 
of  the  eye  to  the  center  of  the  cornea ;  the  visual 
axis    is    a    line   drawn    from    the   object    viewed 
through  the  nodal  point  to  the  yellow  spot.     The 
angle  made,  by  these  two  lines  is  called  the  angle 
alpha,  and  in  the  normal  eye  is  about  5°. 


154  REFRACTION 

The  Angle  Alpha. — In  myopia  the  nodal 
point  is  relatively  further  back  than  in  the  normal 
eye,  and  the  angle  alpha  is  thereby  enlarged. 
This,  by  a  confusion  of  judgment  in  the  observer 
between  the  optic  and  visual  axes,  gives  the  pa- 
tient the  appearance  of  having  a  divergent  stra- 
bismus. In  hypermetropia  the  nodal  point  is  rela- 
tively further  forward  than  normal,  and  the  angle 
alpha  is  diminished,  sometimes  completely  ob- 
literated, and  occasionally  the  two  axes  cross,  in 
which  case  the  angle  alpha  is  said  to  be  negative. 
This  gives  the  patient  the  appearance  of  having 
a  convergent  strabismus. 

It  is  of  course  highly  important  to  be  able  to 
determine  w^hether  a  given  case  of  strabismus  is 
real  or  apjDarent,  and  fortimately  this  is  very  eas- 
ily ascertained.  We  simply  hold  an  object,  such 
as  a  pencil,  about  a  meter  from  the  patient's  eyes, 
directing  him  to  keep  looking  at  it,  and  gradually 
move  it  nearer  to  his  eyes.  If  both  visual  axes 
continue  to  be  directed  toward  the  object  the  case 
is  one  of  apparent  strabismus.  But  if  during  the 
experiment  one  eye  suddenly  deviates,  either  in- 
ward or  outward,  then  we  have  a  case  of  real 
strabismus,  which  must  then  be  diagnosed  by  fur- 
ther means. 

Beal  strabismus  is  due  to  a  defect  in  the  func- 
tion of  the  recti  muscles  of  the  eye,  and  may  be 
generally  classified  into  Concomitant  and  Par- 
alytic strabismus .  Properly  speaking,  these  two 
varieties   arc    but   different    deirrees    of   the    same 


STRABISMUS    AND   IMBALANCE 


155 


troubk',  but  for  practical  purposes  we  recognize 
as  concomitant  strabismus  those  cases  in  which 
the  deviation  of  the  squinting  eye  is  constant, 
and  its  range  of  motion   is  practically  equal  to 


Illustrating  convergent  strabismus. 

that  of  the  sound  eye,  while  by  paralytic  strabis- 
mus we  understand  a  condition  in  which  the  par- 
alysis of  the  recti  is  such  as  to  seriously  interfere 
with  motion  of  the  deviating  eye. 

Diagnosis  of  Strabismus. — In  some  cases  it  is 
possible   to   determine  roughly  between   concomi- 


Illustrating  divergent  strabismus. 

taut  and  paralytic  strabismus,  by  simply  having 
the  patient  follow  with  his  eyes  a  small  object 
moved  in  various  directions,  and  observing  whether 
or  not  the  deviating  eye  follows  the  motions  of 
the  sound  one.     Bnt  this  is  not  at  all  a  reliable 


loO  REFRACTION 

test.  The  only  trustworthy  way  of  diagnosing  the 
nature  of  the  squint  is  as  follows: 

First  instructing  the  patient  to  look  straight 
in  front  of  him,  make  a  mark  on  the  lower  eye- 
lid with  a  piece  of  crayon  indicating  the  position 
of  the  pupillary  center  in  each  eye. 

N'ow  cover  one  eye  in  such  a  manner  as  to  ob- 
struct the  patient's  view,  but  so  that  the  observer 
can  watch  the  movements  of  the  covered  eye.  In- 
struct the  patient  to  look  steadily  with  his  uncov- 
ered eye  at  a  small  object  held  a  short  distance 
from  him.  (This  is  called  "fixing"  the  object.) 
The  covered  eye  will  be  seen  to  make  a  sudden 
movement  inward  or  outward,  and  the  extent  of 
this  movement  should  be  indicated  by  another 
chalk  mark  on  the  eyelid. 

Now  cover  the  other  eye  and  repeat  the  experi- 
ment. If  the  deviating  movements  made  by  the 
two  eyes,  as  indicated  by  the  chalk  marks,  are 
equal,  the  strabismus  is  concomitant.  If  one  eye 
made  a  greater  deviation  than  the  other,  the  stra- 
bismus is  paralytic,  and  that  eye  which  made  the 
greater  deviation  is  the  sound  e^-e. 

The  deviation  made  by  the  squinting  eye  in  this 
experiment  is  called  the  primary  deviation,  that 
made  by  the  sound  eye  the  secondary  deviation, 
and  the  law  is  that  in  concomitant  strabismus  the 
primary  and  secondary  deviations  are  equal ;  while 
in  paralytic  strabismus  the  secondary  deviation  is 
greater  than  the  primary. 


STRABISMUS    AND    IMBALANCE  157 

Farahjtic  strahismus  is  of  course  a  matter  for 
the  neurologist  to  deal  with,  and  need  not  be  con- 
sidered in  a  work  on  optics.  By  far  the  greater 
proportion  of  cases  of  concomitant  strabismus, 
however,  are  due  to  errors  of  refraction,  high  de- 
grees of  hypermetropia  or  myopia,  and  are  there- 
fore legitimate  subjects  for  the  optician. 

Treatment  of  Strabismus. — In  recent  cases, 
and  in  cases  of  what  is  called  periodic  strabis- 
mus, i.  e.,  where  the  squint  appears  only  under 
the  strain  of  near  vision  or  at  times  of  great 
bodily  fatigue,  the  proper  correction  of  the  re- 
fractive error  and  resting  the  eyes  from  near  work 
is  often  sulficient  to  effect  a  cure.  In  more  per- 
manent cases  it  is,  frequently  necessary  to  give 
the  patient  prolonged  rest  for  his  eyes  by  means 
of  atropin,  say  three  or  four  weeks. 

In  divergent  strabismus  much  good  may  some- 
times be  accomplished  by  orthoptic  exercises  by 
means  of  prisms,  for  particulars  of  which,  how- 
ever, a  larger  work  must  be  consulted.  Such  treat- 
ment requires  great  patience  and  judgment,  and 
should  not  be  carried  out  by  one  who  does  not 
thoroughly  understand  the  anatomy  and  physi- 
ology of  the  matter.  For  convergent  strabismus  no 
exercises  of  this  kind  are  serviceable,  as  there  is 
no  known  means  of  exciting  the  external  recti  to 
contraction. 

If  none  of  these   expedients  succeed,   the   case 
becomes  one  for  the  ocular  surgeon. 


158  REFRACTION 

Muscular  Imbalance. 

Of  much  more  frequent  occurrence  than  actual 
fctrabismus  is  the  condition  of  latent  deviation 
known  as  muscular  imbalance^  or  heterophoria, 
in  which  one  of  the  extrinsic  muscles  of  the  eye 
has,  from  some  cause  or  other,  become  less  effi- 
cient than  its  antagonist,  necessitating  an  ex- 
cessive innervation  of  the  weaker  muscle  in  or- 
der to  prevent  deviation,  and  giving  rise  to  whai 
IS  known  as  eye-strain^  with  occasional  lapses  into 
periodic  strabismus. 

Varieties  of  Heterophoria. — According  to 
the  direction  in  which  the  63^6  tends  to  deviate, 
heterophoria  has  been  divided  into  three  general 
classes,  dependent,  of  course,  upon  insufficiency  of 
the  muscle  acting  in  the  opposite  direction,  as 
follows : 

Variety.  Tendency.  Faulty  Muscle. 

Exophoria  Outward  Internal  Eectus 

Esophoria  Inward  External  Eectus 

Hyperophoria      Upward  Obliques. 

Inasmuch,  however,  as  this  classification  in- 
volves an  inversion  of  the  nomenclature  of  the 
condition  and  the  muscle  at  fault,  it  is  not  a  very 
convenient  one,  and  the  writer  much  prefers 
Gould's  method  of  designating  the  trouble  by  the 
term  "imbalance"'  of  the  muscle  in  question. 

Of  the  above  varieties  of  imbalance  the  last  is 
so  rare  that  it  will  be  disregarded  in  the  present 
work. 


STRABISMUS    AND    IMBALANCE  159 

Causes  of  Imbalance. — The  muscular  insuffi- 
ciency may  be  due  to  a  general  lack  of  muscular 
or  nervous  tone,  whose  underlying  cause  is  to  be 
found  in  some  constitutional  disease,  such  as  tu- 
berculosis, syphilis,  anemia,  neurasthenia,  etc. 
Far  more  commonly,  however,  it  is  a  result  of 
long-continued  error  of  refraction,  involving  an 
extraordinary  disturbance  of  the  normal  relations 
between  accommodation  and  convergence.  In 
fact,  every  condition  of  ametropia  inevitably  and 
logically  produces  muscular  imbalance,  and  it  is 
simply  a  question  of  the  degree  in  which  it  exists, 
ranging  all  the  way  from  imperceptible  hetero- 
plioria  to  actual  strabismus. 

Rationale  of  Refractive  Imbalance. — It  has 
already  been  explained  that,  while  accommodation 
and  convergence  are  anatomically  distinct  and  sep- 
arate functions,  they  are  very  intimately  asso- 
ciated, and  the  exercise  of  one  is  a  powerful  stim- 
ulus to  the  performance  of  the  other.  Hence 
when,  as  in  hypermetropia  or  myopia,  the  normal 
relation  between  the  two  functions  is  disturbed 
by  tlie  abnormal  conditions  of  accommodation, 
the  natural  tendency  of  convergence  is  to  follow 
suit,  and  it  is  only  prevented  from  doing  so  by 
an  excessive  innervation  of  the  muscle  concerned 
in  such  prevention. 

Thus,  .a  hypermetrope,  as  we  have  seen,  is 
obliged  to  use  his  accommodation,  for  objects  at 
infinity,  and  wlicii  he  does  so  the  natural  ten- 
dencv  of  the  internal  recti  is  to  contract  and  con- 


160 


REFRACTION 


\erge  the  eyes  in  proportion  to  the  degree  of  ac- 
commodation exerted.  But  if  this  were  done,  con- 
vergence for  distant  vision  would  result,  whereas 
the  imperative  desire  of  the  brain  is  for  single 


Illustrates  the  two  dissimilar  images  seen  through 
the  Maddox  rod. 

vision.  Therefore  an  extraordinary  innervation 
is  applied  to  the  external  recti  to  prevent  con- 
vergence. 

A  myope,  on  the  other  hand,  does  not  accom- 
modate for  near  objects,  which  nevertheless  de- 
mand convergence  in  order  to  produce  a  single 
image.  The  normal  stimulus  to  the  internal  recti 
is  here  lacking,  and  the  external  recti  are  nat- 
urally stimulated  to  contract  in  correspondence 
witli  the  negative  accommodation.  An  excessive 
innervation  is  therefore  necessary  to  bring  the 
internal  recti  into  adequate  play. 

MusciFLAR  Failure. — Tn  both  of  these  condi- 


STRABISMUS    AND    IMBALANCE 


1(51 


tions,  if  long  continued  and  the  eyes  constantly 
used,  th^  overworked  muscle  eventually  becomes 
weakened,  and  there  is  a  latent  deviation  toward 
the  direction  of  its  antagonist,  which  is  only  pre- 
vented by  an  increasing  conscious  effort  which  at 


Illustrates  the  fusion  of  images  as  seen  by  the  nor- 
mal eye  through  the  Maddox  rod. 

times  (especially  under  strain  of  bodily  fatigue) 
fails,  and  temporary  strabismus  occurs.  In  hyper- 
metropic imbalance  (by  far  the  commonest  tyi3e) 
it  is  the  external  recti  that  suffer  and  the  ten- 
dency is  to  convergence;  in  myopic  imbalance  the 
internal  recti  are  overworked,  and  the  tendency 
is  to  divergence. 


162 


REFRACTION 


DlSTINCTlOX  BeTWELIT  IMBALANCE  AND  STRA- 
BISMUS.— Eventually,  of  course,  imbalance,  if  not 
relieved,  will  end  in  strabismus,   which  is  func- 


Illustrates  the  dissociated  images  as  seen  in  di\'Terg- 
ent   imbalance   with  the  Maddox   rod. 


tionally  only  an  intenser  degree  of  heterophoria. 
The  practical  difference  between  the  two  condi- 
tions is  that  in  imbalance  the  brain  is,  by  an 
effort,  maintaining  single  vision  at  the  expense 
of  the  muscle,  whereas  in  strabismus  the  muscle 
is  no  longer  able  to  maintain  proper  poise;  single 
vision  is  then  impossible,  therefore  the  brain  has 
ceased,  to  strive  for  it,  but  fixes  the  object  with 
one  eye  or  tlie  other,  disregarding  tlio  image  on 
the  unused  eve. 


STRABISMUS    AND    IMBALANCE 


163 


Tests  for  Imbalance.— These  are,  in  the  gross, 
the  same  as  those  already  given  for  strabismus, 
dependent  upon  the  inability  of  the  affected  eye 
to  accurately  follow  the  convergent  movements 
of  the  sound  eye. 

Maddox  Rod. — A  much  more  delicate  and  re- 
liable test,  however,  is  -that  furnished  by  the 
^laddox  rod  found  in  every  trial  case.  This  de- 
vice is  an  opaque  disc  into  which  is  set  a  cylin- 
drical piece  of  glass,  which  is  mounted  before  the 
suspected  eye,  the  other  eye  being  furnished  with 


Illustrates    the    dissociated    images    as    seen    in    con- 
vergent Imbalance  with   the  Maddox  rod. 

a  colored  piano  lens,  and  the  patient  instructed  to 
look  at  a  small  flame  six  meters  away. 

The  principle  of  the  Maddox  test  is  that  the 


164  RE  FB  ACTION 

image  of  the  flame  on  the  uncovered  eye  is  that 
of  a  round  colored  flame,  while  on  the  covered 
eye  it  is  drawn  out  into  a  long  bar  of  white  light, 
thus  dissociating  the  retinal  conceptions  of  the 
image  and  lessening  the  desire  for  single  vision. 


Maddox   Rod. 

If  the  muscles  are  in  perfect  balance  the  images 
on  the  two  retinae  will  accord  without  effort  and 
irrespective  of  the  identity  or  difference  of  their 
form,  and  the  round  colored  flame  will  be  seen 
with  the  rod  of  white  light  running  through  it. 

If  there  is  muscular  imbalance,  the  dissocia- 
tion of  the  retinal  images  and  the  consequent 
weakening  of  the  desire  for  single  vision  will 
induce  the  patient  to  give  up  his  effort  to  pre- 
serve the  single  image;  double  vision  will  at  once 
result,  and  the  rod  of  white  light  A\ill  l)e  'seen 
to  one  side  of  the  colored  flame — on  the  same  side 
as  the  covered  eye  if  the  imbalance  is  convergent, 
on  the  opposite  side  if  divergent. 


STKAniSMUS    ANIJ    IMIIAJ.ANCE 


165 


Use  of  the  Maddox  Rod. — It  is  important  that 
the  disc  containing  the  Maddox  rod  be  placed 
exactly  in  front  of  the  patient's  pupil,  otherwise 
it  will  entirely  fail  of  its  effect,  i  It  must  also 
be  remembered  that  as  the  rod  is  a  cylinder  it 
will  draw  out  the  flame  into  a  ])ar  precisely  at 
right  angles  to  the  axis  of  the  rod,  and  must 
therefore  be  placed  before  tlie  eye  at  right  angles 
to  the  direction  in  wliieh  it  is  desired  that  the  bar 
of  light  shall  appear.  In  testing  for  convergent 
and   diverii-ent   imbalance^    (tlic  oiilv   Iwo   varieties 


Showing    how    the    Maddox    rod    draws    out    a    bar    of 
lig-ht  at  rig-ht  angles   to  its  axis. 

here  considered),  it  is  desirable  to  have  the  rod 
appear  vertical,  hence  the  rod  should  be  placed 
horizontally.  In  testing  for  hyperophoria,  or  im- 
balance of  the  obliques,  the  reverse  is  desirable. 
This  form  of  imbalance,  however,  is  rare,  and  has 
no  direct  relation  to  refraction. 


166  REFRACTION 

Prism  Test. — In  cases  of  bilateral  muscular 
insufficiency,  which,  of  course,  can  hardly  be  re- 
garded as  imbalance,  since  both  eyes  are  alike 
affected,  the  insufficiency  is  detected  and  measured 
by  means  of  prisms,  as  described  under  Con- 
vergence. 

Eliminatiox  of  Refractive  Errors. — As 
stated,  every  condition  of  ametropia  is  inherently 
attended  by  muscular  imbalance,  hence  by  the  time 
the  patient  comes  to  the  refract ionist  a  part  of 
the  muscular  trouble  has  become  permanent,  due 
to  anatomical  changes  in  the  muscle,  and  a  part 
of  it  is  still  an  integral  factor  in  the  ametropia. 
After  testing  the  amount  of  imbalance  in  the  un- 
aided eye,  therefore,  the  operator  should  correct 
the  error  of  refraction  with  appropriate  lenses, 
and  make  another  test,  subtracting  the  result  of 
the  second  test  from  tliat  of  the  first  to  find  the. 
net  amount  of  ])ormanent  imbalance  which  needs 
treatment. 

Treatment  of  Imbalance. — If  the  degree  of 
heterophoria  is  slight  it  is  usually  sufficient  to 
correct  the  error  of  refraction,  and  when  this 
cause  of  the  trouble  is  removed  the  muscle  will 
right  itself.  In  severer  cases,  however,  a  coursi^ 
of  optic  exercises  must  be  carried  out,  with  prisms 
base  in  or  base  out  as  the  case  may  demand,  and 
as  indicated  by  what  has  already  been  said.  These 
exercises  should  be  nicely  graduated,  and  care- 
fully supervised  by  the  refractionist,  and  need  to 
be  persisted  in  witli  great  constancy  and  patience. 


STRABISMUS    AM)    IMBALANCE  1()7 


111  cases  of  eoiivergeiit  iiiibalaiice  the  results  are 
usually  veiT  satisfactory;  in  divergent  imbalance 
they  are  less  encouraging  as  there  is  no  known 
stimulus  foi"  the  unilateral  contraction  of  the  ex- 
ternal rectus. 


CHAPTER  XIV. 
ASTHENOPIA. 

Asthenopia  is  a  broad  term  used  to  designate 
tliat  group  of  symptoms  which  results  from  any 
form  of  eye  strain  due  to  functional  causes,  in- 
cluding that  type  of  muscular  and  nervous  ex- 
haustion which  has  already  been  dealt  with  at 
length  in  the  chapter  on  Muscular  Imbalance. 
In  a  general  way  these  symptoms  are  the  same, 
from  whatever  specific  cause  they  arise,  and  in 
the  last  analysis  they  are  essentially  reflex  in  their 
character,  dependent  upon  an  excessive  and  un- 
v-^qual  innervation,  and  mediated  primarily  through 
the  ocular  and  facial  nerves. 

Sympto^es  of  Asthenopia. — Asthenopia  mani- 
fests itself  by  an  inability  to  sustain  a  steady 
and  prolonged  convergence,  and  by  more  or  less 
pain  in  the  eye  when  this  is  attempted.  It  is  an 
exceedingly  common  condition,  so  much  so  that 
the  presence  of  pain  in  the  eyes,  with  no  other 
symptoms  of  inflammation,  immediately  suggests 
asthenopia.  As  a  rule  the  pain  is  not  very  severe, 
although  it  may  reach  the  point  of  agonizing  neu- 
ralgia. Sometimes  there  is  no  actual  pain  at  all, 
but  after  prolonged  use  of  the  eyes  the  vision  be- 
comes indistinct,  and  the  patient  is  obliged  to 
<io])  his  work  and  rest  his  eyes  by  relaxing  their 
accommodation  and  convergence. 

If  by  an  effort  the  patient  compels  himself  to 
continue  his  work  in   spite  of  the  inconvenience. 


17U  REFRACTION 

he  will  presently  begin  to  develop  positive  symp- 
toms of  inflammation,  as  photonliobia,  conjunc- 
tivitis, etc.  Headache  is  a  very  prominent  feature 
of  asthenopia,  and  assumes  various  forms.  Usu- 
ally it  is  that  of  a  heaviness  or  pain  across  the 
brow,  similar  to  that  produced  by  nasal  catarrh. 
Often  there  is  pain  in  the  back  of  the  neck,  espe- 
cially if  due  to  astigmatism,  and  there  are  vari- 
ous tender  spots  upon  the  head.  Hence  this  train 
of  symptoms  should  always  suggest  an  examina- 
tion of  the  refraction  of  the  e3^es. 

Varieties. — As  already  stated,  a  common  form, 
of  asthenopia  is  that  which  results  from  muscular 
imbalance,  by  reason  of  the  unequal  and  excessive 
innervation  performed  by  the  patient  in  order  to 
preserve  single  vision.  This  type  of  the  complaint 
is  generally  classified  as  muscular  asthenopia,  in- 
dicating that  its  anatomical  and  physiological  seat 
is  in  the  extrinsic  ocular  muscles,  and  is  often 
due  to  prolonged  work  at  near  distance. 

Myopia  is  also  frequently  responsible  for  this 
variety  of  asthenopia,  on  account  of  the  extra 
amount  of  convergence  that  has  to  be  exerted  and 
the  consequent  straining  of  the  internal  recti 
muscles. 

A  second  form  of  asthenopia,  which  really  be- 
longs to  the  muscular  type,  is  that  which  arises 
from  straining  of  the  ciliary  muscle  in  cases  of 
hypermetropia  or  hypermetropic  astigmatism.  For 
the  sake  of  distinction  from  that  which  depends 
upon  the  extrinsic  muscles,  however,  this  variety 


ASTHENOPIA 


171 


is  usually  known  as  accommodalicc  asthenopia. 
It  is  by  far  the  most  common  form  of  the  trouble, 
because  hypermetropia  and  hypermetropic  astig- 
matism are  by  far  the  most  common  errors  of 
refraction,  and  furthermore  it  is  the  type  of  asthe- 
nopia which  gives  most  trouble  to  the  refraction- 
ist,  because  the  irritable  and  spasmodic  condition 
of  the  ciliary  muscle  renders  it  almost  impossible 
to  obtain  reliable  results  from  the  various  tests 
by  which  he  endeavors  to  detect  and  measure  the 
refractive  errors,  almost  all  of  which  are  based 
upon  the  function  of  the  accommodation. 

When  such  a  condition  exists,  it  usually  evi- 
dences itself  very  quickly  under  examination  by 
the  patient  giving  erratic  replies,  seeing  first  one 
thing  and  then  another,  or  by  an  excessively  sen- 
sitive contraction  of  the  pupil  under  illumina- 
tion so  that  the  observer  cannot  get  a  satisfactory 
view  of  the  fundus,  and  in  such  cases  it  is  neces- 
sary to  either  prescribe  absolute  rest  of  the  eye 
for  several  days  before  continuing  the  examina- 
tion or  to  paralyze  the  muscle  with  atropin  or  by  a 
strong  convex  lens. 

From  the  patient's  standpoint  this  form  of 
asthenopia  is  the  most  annoying  and  distressing, 
because  its  cause  is  so  frequently  overlooked  by 
both  medical  men  and  by  refractionists.  It  should 
l)e  borne  in  mind  that  a  very  slight  amount  of 
astigmatism  left  uncorrected,  even  though  the 
greater  portion  of  it  has  been  corrected,  is  suffi- 
cient to  give  rise  to  the  symptoms  of  accommo- 


172  REFRACTION 

dative  asthenopia.  Great  care  should  therefore  be 
taken  in  correcting  astigmatism  to  do  it  accurately 
and  thoroughly. 

A  third  and  rarer  type  of  astlienopia  is  that 
which  comes  from  sensitiveness  or  fatigue  of  the 
retina,  due  either  to  close  and  continuous  work 
in  bright  light  or  over  glittering  materials  or  to 
constitutional  diseases.  This  is  called  retinal 
asthenopia. 

Reflex  Effects  of  Eye  Straix. — An  intelli- 
gent understanding  of  the  reflex  effects  of  eye 
strain  of  course  demands  an  intelligent  pre-con- 
ception  of  the  nature  of  what  is  meant  by  eye 
strain,  and  that  again  requires  a  more  or  less  ex- 
tensive knowledge  of  the  nervous  and  muscular 
mechanism  of  the  eye  and  of  the  function  of 
vision. 

Every  one  knows,  in  a  general  way,  that  the 
muscular  activity  of  the  body  is  under  the  rule 
of  the  nervous  system;  tliat  the  muscles  are  fur- 
nished with  their  motive  power  from  the  nerve 
centers  (in  the  brain  and  spinal  cord)  by  mean? 
of  conducting  fibres,  commonly  called  nerves,  and 
that  each  muscle,  or  group  of  muscles,  is  supplied 
by  a  separate  nerve,  or  group  of  nerves,  usually 
originating  from  a  distinct  and  separate  center  in 
the  brain  or  cord. 

In  a  still  vaguer  fashion  every  one  knows  that 
this  great  system  of  nerve  fibres  and  nerve  cen- 
ters cross  and  re-cross,  exchanging  small  fibrils 
with  each  other,  like  some  vast  telephone  or  tele- 


ASTHENOPIA  173 

graph  system  with  "crossed  wires/^  so  that  all 
the  centers  and  all  the  out-points  (called  "periph- 
eries") are  intimately  connected  one  with  the 
other;  none  is  isolated  or  independent  of  another, 
but  while  a  nervous  current  may  be  sent  by  a 
special  center,  along  a  special  set  of  fibres  to  a 
special  group  of  muscles  to  perform  a  special  act, 
and  in  such  case  the  special  set  of  nerves  and 
muscles  concerned  get  the  gi-eat  bulk  of  the  nerv- 
ous current,  yet  every  nerve  center  and  every  organ 
in  the  body  shares  to  some  extent  in  the  nervous 
discharge. 

Co-ordination. — But  what  is  not  so  generally 
known  among  laymen  is  that  in  the  furnishment 
of  this  motive  power  by  the  nerve  centers  under 
normal  conditions  there  is  a  more  or  less  eco- 
nomic regulation  of  the  amount  of  nerve  energy 
furnished  in  accordance  with  the  amount  of  work 
to  be  done.  Especially  is  this  the  case  in  those 
functions  which  are  more  or  less  outside  the  pale 
of  consciousness — which  are  what  we  call  auto- 
matic— and  in  those  *\'ery  fine,  delicate  move- 
ments which  require  nicety  of  iudgment. 

Nature,  left  to  herself,  as  in  the  case  of  auto- 
matic function,  never  wastes  an  iota  of  nervous 
energy;  for  the  performance  of  each  act  just  suffi- 
cient nerve  energ}^  is  discharged  to  properly  ac- 
complish it  and  no  more;  and  when  conscious 
movements  are  to  be  very  finely  adjusted  as  in 
threading  a  needle  or  picking  up  a  pin  the  same 
nervous   economy   has   to   be   practiced.     Usually 


174  REFRACTION 

two  sets  of  muscles  have  to  be  brought  into  play 
for  every  act,  one  set  opposing  the  other,  and 
the  motive  force  supplied  to  each  set  is  nicely 
balanced  so  as  to  give  just  the  needed  resultant 
effect.    This  is  called  co-ordination. 

Abnormal  Accommodation.^ — Now  the  func- 
tions of  accommodation  and  convergence  both  be- 
long to  this  class  of  function ;  usually  largely  auto- 
matic, but  whether  automatic  or  conscious,  always 
requiring  very  nice  co-ordination  of  nervous  en- 
ergy. 

Under  normal  conditions,  that  is  to  say,  when 
the  conditions  of  accommodation  and  convergence 
are  the  same  in  both  eyes,  the  muscular  and  nerv- 
ous mechanism  is  exactly  symmetrical — precisely 
the  same  amount  of  motive  power  is  required  for 
the  extrinsic  muscles  of  each  eye  in  order  to  hold' 
the  two  eyes  balanced  in  proper  convergence,  and 
the  same  for  each  sphincter  to  maintain  equally 
focused  images  on  the  two  retinae. 

But  suppose  that  asinometropia  exists  (far  the 
commonest  cause  of  eye  strain)  and  the  accommo- 
dative conditions  for  the  eyes  are  different.  Then 
the  amount  of  muscular  effort  to  be  exerted  by  one 
ciliary  muscle  is  no  longer  equal  to  that  required 
in  the  other,  and  the  innervation  of  the  two  eyes 
is  no  longer  symmetrical. 

In  order  to  satisfy  the  imperative  demand  of 
the  individual  for  clear  vision,  and  focus  the  rays 
in  both  eyes  with  equal  clearness,  one  ciliary 
muscle   must   contract   more   than    the   other;   an 


ASTHENOPIA 


luiiiatural  and  unequal  amount  of  nerve  energy 
must  be  supplied  by  the  brain  to  one  of  the  cil- 
iaries.  In  other  words,  co-ordination  becomes  an 
unnatural  and  extraordinary  effort,  much  the  same 
as  though  a  man  were  continually  endeavoring 
to  balance  a  weight  in  a  difficult  and  unstable 
position. 

Abnormal  Convergence. — On  the  other  hand, 
suppose  tlie  conditions  of  convergence  are  abnor- 
mal. Although  the  functions  of  convergence  and 
accommodation  are  really  distinct  and  separate 
functions,  they  are  very  intimately  associated,  and 
the  exercise  of  one  is  a  powerful  stimulus  to  the 
performance  of  the  other.  Hence  when,  as  in 
hypermetropia  or  myopia,  the  normal  relation  be- 
tween the  two  functions  is  disturbed  by  the  al)- 
uormal  conditions  of  accommodation,  the  natural 
tendency  of  convergence  is  to  follow  suit,  and  it 
is  only  prevented  from  doing  so  by  an  excessive 
innervation  of  tlie  muscle  concerned  in  such  pre- 
vention. 

Hypermetropia. — Thus,  a  hvpermetrope,  as  we 
liave  seen,  is  obliged  to  use  his  accommodation 
for  objects  at  infinity,  and  when  he  does  so  the 
natural  tendency  of  the  internal  recti  is  to  con- 
tract and  converge  the  eyes  in  proportion  to  the 
degree  of  accommodation  exerted.  But  if  this 
were  done,  convergence  for  distant  vision  would 
result,  whereas  tlie  imperative  desire  of  the  l)rain 
is  for  sinofle  vision.     Therefore  an  extraordinarv 


176  REFRACTION 

innervation  is  applied  to  the  external  recti  to  pre- 
vent convergence. 

Myopia. — A  myope^  on  the  other  hand,  does 
not  accommodate  for  near  objects,  which  never- 
theless demand  convergence  in  order  to  produce 
a  single  image.  The  normal  stimulus  to  the  in- 
ternal recti  is  here  lacking,  and  the  external  recti 
are  naturally  stimulated  to  contract  in  corre- 
spondence with  the  negative  accommodation.  An 
excessive  innervation  is  therefore  necessary  to 
bring  the  internal  recti  into  adequate  play. 

In  either  of  these  abnormal  conditions,  i.  e.,  of 
accommodation  or  of  convergence,  the  nervous 
eifort  required  to  balance  the  muscles  to  the  de- 
sired resultant  of  function,  is  unnatural  and  ex- 
traordinary. 

Now,  as  already  explained,  the  whole  nervous 
organization  of  the  body  shares  in  the  nervous 
discharge  of  any  one  function  and  possibly  this 
is  so  to  an  especial  degree  in  the  case  of  vision, 
-which  is  so  essential  a  factor  in  every  action, 
emotion,  and  thought.  Hence  any  unnatural  and 
extraordinary  exercise  of  the  nervous  mechanism 
of  vision  cannot  but  make  itself  very  quickly  felt 
in  an  unnatural  and  extraordinary  nervous  im- 
pression upon  other  organs  and  functions  of  the 
body. 

(lOuld  has  shown  that  the  organs  and  func- 
tions most  closely  associated  with  the  ocular  nerves 
are  those  of  digestion,  hence  the  commonest  dis- 
turbances resulting  from  eye  strain  are  those  of 


ASTHENOPIA  177 

stomach  and  bowels;  lack  of  appetite,  dyspepsia^, 
nausea,  vomiting,  constipation,  etc.  Next  to  these 
come  disturbances  of  the  general  nervous  system, 
chorea,  bed-wetting,  insomnia,  and  even  epilepsy. 

Dizziness  is  always  a  more  or  less  noticeable 
accompaniment  of  eye  strain,  because  it  is  by 
the  unconscious  estimate  we  make  of  the  amount 
of  nerve  energy  expended  in  accommodation  and 
convergence  that  we  largely  judge  of  distances, 
sizes,  shapes,  etc.,  and  when  this  faculty  is  un- 
naturally exercised  dizziness  results. 

Thus  it  is  that  many  cases  of  digestive  and 
nervous  disease  are  caused  by  eye  strain  and  may 
be  cured  by  the  proper  fitting  of  glasses,  which 
correct  the  refractive  errors,  render  the  eyes  nor- 
mal and  equalize  innervation.  And  in  this,  too, 
one  at  once  perceives  the  ultra-importance  of  care- 
ful and  accurate  estimation  and  correction  of  re- 
fractive errors,  not  only  to  cure  eye  strain,  but  to 
avoid  producing  it. 


CHAPTER  XV. 

DISEASES     OF     THE     EYE     CONNECTED 
WITH  DISTURBANCES  OF  VISION. 

As  a  general  proposition  it  may  be  stated  that 
if,  after  a  thorough  and  conscientious  applica- 
tion of  the  methods  herein  set  forth,  the  patient's 
acuity  of  vision  is  not  improved,  the  trouble  is 
due  to  other  than  refractional  errors,  and  is  a 
proper  subject  for  medical  or  surgical  attention. 
However,  it  is  well  to  be  able,  as  the  examination 
proceeds,  to  recognize  pathological  conditions  by 
the  same  agencies  and  means  as  are  being  used 
to  detect  errois  of  refraction,  and  with  a  little 
intelligent  care  this  can  be  readily  done. 

A  great  many  of  the  commoner  diseases  of  the 
eye  can  be  at  least  suspected,  if  not  recognized, 
from  unaided  observation.  More  definite  infoi'- 
mation  can  in  many  cases  be  obtained  by  means  of 
the  ophthalmoscope,  and  for  this  reason,  if  for 
no  other,  it  is  advisable  that  an  objective  exam- 
ination be  made  of  every  eye  that  comes  to  one  for 
correction.  A  very  little  intelligent  practice  will 
enable  the  observer  to  detect  and  identify  abnor- 
mal conditions  of  the  fundus  seen  through  the 
mirror. 

Following  are  some  of  the  diseases  of  the  eye 
most  commonly  met  with  in  connection  with  diffi- 
culties of  vision,  for  which  the  patient  consults 
the  refractionist. 


180  REFRACTION 

Simple  Acute  Conjunctivitis. 

This  occurs  to  a  more  or  less  extent  in  almost 
every  ametropic  e3^e,  due  to  congestion  <3onse- 
quent  upon  eye  strain,  and  is  the  commonest 
trouble  in  the  refractionist's  experience.  It  is 
highly  important  that  he  be  able  to  differentiate 
it  from  more  serious  and  deep  seated  diseases. 

Symptoms. — In  its  mild  forms  it  is  character- 
ized by  more  or  less  injection  of  the  ocular  con- 
junctiva (i.  e.;  the  conjunctiva  covering  the  eye- 
ball), itching  and  burning,  sensation  as  of  sand 
in  the  eye,  stickiness  of  the  lids,  and  sensitive- 
ness to  light.  In  its  severer  forms  all  of  these 
symptoms  are  intensified,  and  the  discharge,  which 
is  thick,  is  so  profuse  as  to  glue  the  lids  together 
when  closed  for  any  length  of  time. 

Diagnosis. — The  characteristic  feature  about 
conjunctivitis  is  the  superficiality  of  the  conges- 
tion, as  shown  by  the  brick  red  color  of  the  in- 
jected vessels,  which  run  in  a  tortuous  network, 
whose  individual  branches  can  be  readily  distin- 
guished, and  can  be  moved  witli  the  membrane. 
No  other  disease  of  the  eye,  producing  injected 
vessels,  shows  this  characteristically  superficial  set 
of  conditions. 

Treat  MEXT. — The  milder  forms,  dependent 
upon  errors  of  refraction,  need  only  a  correction 
of  the  latter  to  remedy  the  congestion.  Cloths 
wrung  out  of  cold  water  make  a  very  grateful 
application.  In  the  more  severe  forms  the  eye 
should  be   protected   from   the  light,   and   a  sat- 


DISEASES    OF    THE    EYE  131 

urated  solution  of  boric  acid  instilled  at  frequent 
intervals. 

Chronic  Conjunctivitis 

Presents  the  same  set  of  symptoms  as  the  acute 
variety,  but  in  more  obstinate  and  subdued  form. 
The  lids  and  conjunctiva  are  usually  thickened. 

TREATiMENT. — Daily  instillations  of  astringent 
lotions,  as  tannic  acid  or  nitrate  of  silver,  % 
grain  to  the  ounce,  ointment  of  yellow  oxid  of 
mercury,  and,  the  ap])lication  of  cojipor  sulphate 
stick  once  a  week.  Errors  of  refraction,  if  pres- 
ent, must  be  corrected,  or  a  cure  will  never  be 
effected. 

Purulent  or  Gonorrheal  Conjunctivitis 

rarely  comes  under  the  notice  of  the  refraction- 
ist.  It  is  a  very  virulent  and  rapid  inflammation, 
due  to  gonococcus  infection. 

Symptoms. — Those  of  acute  conjunctivitis,  but 
in  a  most  violent  form,  rapidly  passing  into  pro- 
fuse suppuration.  Pain  is  intense,  and  extends 
to  the  surrounding  parts.  There  is  great  swell- 
ing. The  disease  has  been  known  to  destroy  an 
eye  within  twenty-four  hours. 

Treatment,  whieli  is  urgent,  should  be  under- 
taken only  by  a  skilled  oculist.  The  all-important 
factoid  in  "first  nid"  is  \o  ])romptly  protect  the 
sound  eye,  by  as  efficient  a  covering  as  possible, 
from  possibility  of  infection,  and  to  thoroughlv 
disinfect  all  of  the  refractionist's  belongings  that 
have  come  in  contact  with  the  patient. 


182  ,  REFRACTION 

Trachoma 

is  a  very  dangerous  and  contagious  form  of  fol- 
licular conjunctivitis,  characterized  by  granular 
hypertrophy  of  the  membrane  and  subsequent 
degeneration  of  tissue. 

Symptoms. — Intense  itching,  burning  and  pain, 
intolerance    of    light,    profuse    waterv    secretion 


Illustrating   the    method   of   bandaging   an    eye. 

(which  is  contagious),  visual  disturbance.  The 
palpebral  conjunctiva  (i.  e.,  the  coniunctiva  cov- 
ering the  lids)  above  and  below  is  thickened  and 
angry-looking,  covered  with  coarse  red  granules, 
giving  the  eye  a  heavy  appearance.  There  are  no 
ophthalmoscopic  phenomena, 

Pannus. 

A  frequent  result  of  trachoma,  is  a  layer  of 
newly-formed  vascular  tissue,  overlying  the  ocu- 
lar conjunctiva  like  an  opaque  skin. 

Treatment  requires  considerable  judgment  and 
])atience,  and  should  be  carried  out  only  by  an 
expert    oculist,    to    whom    the    patient    must    be 


DISEASES    OF    THE    EYE  183 

promptly  referred.  The  refractionist  should  take 
disinfecting  measures. 

Interstitial  Keratitis 

is  a  not  infrequent  disease  coming  under  the  ob- 
servation of  the  refractionist,  and  has  to  be  care- 
fully differentiated  from  conjunctivitis.  It  is  an 
inflammation  of  the  cornea. 

Symptoms. — Gray,  steamy  condition  of  the 
cornea,  injection  of  the  vessels,  watery  secretion, 
intolerance  of  light,  pain,  interference  with  vision. 

Diagnosis. — The  characteristic  feature  about 
the  congestion  of  keratitis  is  that  the  vessels  in- 
volved are  the  deep  ciliary  vessels,  not  the  super- 
ficial ones  of  the  conjunctiva.  The  injection  is 
therefore  seen  as  a  faint  pink,  the  separate  veins 
cannot  be  distinguished,  but  appear  as  a  set  of 
regular  parallel  lines  of  color,  and  cannot  be 
moved  with  the  conjunctiva.  The  steamy  ap- 
pearance of  the  cornea,  the  pain,  and  the  marked 
disturbance  of  vision  further  serve  to  distinguish 
it  from  conjunctivitis. 

Treatment.  —  Protection  from  light,  and 
prompt  reference  to  an  oculist. 

Iritis 

is  an  inflammation  of  the  iris,  usually  due  to  some 
constitutional  disease,  such  as  syphilis,  rheuma- 
tism, tuberculosis,  gout,  diabetes,  etc.,  but  may 
arise  from  any  cause. 

Symptoms. — Pain,  intolerance  of  light,  watery 
secretion,   disturbance  of  vision,  the  iris  is  dull 


184  REFRACTION 

and  lustreless,  the  pupil  contracted  and  irregular, 
owing  to  adhesions  of  the  iris  to  the  lens.  The 
cornea  is  turbid  and  there  is  the  characteristic 
pink  ciliary  injection.  The  pupil  contracts  very 
sluggishly. 

Diagnosis. — The  diagnostic  features  of  iritis 
are  the  pain,  the  appearance  of  tlie  iris,  and  the 
very  marked  dimness  of  vision.  The  pain  is  sharp 
and  neuralgic,  usually  worse  at  night,  and  accom- 
panied bv  tenderness  of  the  eyeball,  due  to  ciliary 
involvement. 

Treatment. — Rest  and  protection  from  light, 
and  prompt  reference  to  a  medical  man. 

Choroiditis. 

Inflammation  and  degeneration  of  tlie  choroid, 
which  is  really  an  extension  of  the  iris. 

Symptoms. — Dimness  of  vision,  distortion  of 
objects,  spots  before  the  eyes,  flashes  of  light  and 
circles,  no  pain. 

Diagnosis. — There  are  no  external  phenomena, 
but  the  ophthalmoscope  shows  yellowish  white 
patches  diffused  over  the  fundus,  with  the  retinal 
arteries  passing  over  them.  Later,  when  atrophy 
of  the  choroid  begins,  the  atrophic  areas  show  as 
white  spots,  and  the  disc  frequently  presents  a 
yellowish  white  color. 

Treatment. — The  same  as  for  iritis. 

Glaucoma 

is  one  of  the  most  insidious  diseases  of  the  eye, 
and  most  often  mistaken  for  other  conditions  un- 


DISEASES    OF    THE    EYE 


185 


til  too  late  to  remedy.  It  is  therefore  highly 
important  that  the  refractionist  should  be  able 
to  recognize  it  early.  It  is  characterized  by  an 
increased  tension  in  tlie  eyeball  and  may  be  sec- 
ondary to  somo  otluM-  ocular  disease  or  primary. 


Illustrates  cupping  of  disc  in  glaucoma,  as  seen  with 
ophthalmoscope. 

Symptoms. — The  first  symptom  is  always  dim- 
ness of  vision,  usually  appearing  in  the  form  of 
a  sensation  of  fogginess,  with  concentric  rings 
around  the  object  looked  at.  This  is  caused  by 
cloudiness  of  tlie  cornea,  wliich  can  be  observed 
if  carefully  looked  for.  The  tension  of  the  eye- 
ball is  increased,  and  it  is  hard  to  the  touch.  The 
pupil  is  dilated,  often  oval,  and  contracts  slug- 
gishly.    There  is  frequently  ciliary  injection.     Ac- 


186  REFRACTION 

commodatiou  is  diminished^  so  that  the  patient 
always  requires  stronger  lenses  than  are  natural 
at  his  age.     There  is  severe  pain. 

Diagnosis. — The  diagnostic  features  are  the  in- 
creased tension  of  the  eyeball,  the  peculiar  aberra- 
tions of  vision,  the  abnormal  diminution  of  ac- 
commodating power,  and  the  pain,  which  is  steady 
in  character.  The  ophthalmoscope  shows  the  disc 
excavated ;  this  is  due  to  the  intraocular  pressure. 
This  phenomenon  is  diagnostic  of  glaucoma. 

Treatment  is  purely  surgical,  and  must  be  un- 
dertaken only  by  a  skilled  oculist. 

Retinitis. 

An  inflammation  and  degeneration  of  the  retina 
usually  due  to  constitutional  diseases. 

Symptoms. — Disturbance  of  vision,  especially 
noticeable  at  night.  Diminution  of  appreciation 
of  light.  Pain  is  rare,  but  there  is  an  uncomfort- 
able feeling  in  tlie  eyes.  There  may  be  intolerance 
of  light. 

Diagnosis. — There  are  no  external  signs.  The 
ophthalmoscope  shows  the  retina  dull,  the  disc 
congested  and  indistinct,  vessels  tortuous  and 
swollen,  discrete  or  confluent  white  or  yellow  spots 
(exudations)  diffused  over  the  retina.  In  the 
retinitis  of  Bright's  disease  (retinitis  albumi- 
nuria) the  white  spots  are  arranged  in  star-shaped 
figures  chiefly  around  the  yellow  spot. 

Treatment. — Rest,  protection  from  light.  The 
refractionist  should  suggest  to  the  patient,  with- 


DISEASES    OF    THE    EYE 


187 


out  unduly  alarming  him,  the  advisability  of  con- 
sulting a  physician,  wlio  will  discover  the  trouble 
bv  urinalvsis. 


lUustrates  appearance  of  fundus  in  aUjuminuric  ret- 
initis,  as   seen    with    ophthannoscope. 


Detached  Retina  is  characterized  subjectively 
by  disturbance  of  the  field  of  vision,  and  the  oph- 
thalmoscopic phenomenon  is  that  of  a  part  of  the 
retina  missing  from  the  field  of  the  mirror.  The 
treatment  is  purely  surgical. 

Optic  Neuritis. 

{Choked  Di^c)    is  an  inllamt'd  condition  of  the 


188  REFRACTION 

optic  nerve^  clue  to  some  brain  or  constitutioraal 
trouble. 

Symptoms. — Disturbance  of  vision.     Xo  pain. 

Diagnosis.  —  The  ophthalmoscope  shows  the 
head  of  the  disc  swollen  and  whitish  in  color,  its 
margins  indistinguishable  and  only  recognized  by 
the  emergence  of  the  vessels,  Avhich  are  themselves 
altered  and  interrupted. 

Treatment. — Sheerlv  the  treatment  of  the  un- 
derlying lesion.     Refer  to  a  neurologist. 
Optic  Atrophy. 

x4.trophy  of  tlie  optic  nerve,  due  to  affections  of 
the  nerve  or  the  retina. 

Symptoms. — Diminution  of  acuteness  of  vision, 
contraction  of  the  field  of  vision,  and  eventual 
blindness. 

Diagnosis. — By  ophthalmoscope  only.  The 
disc  has  a  dense  grayish  white  color,  its  minute 
vessels  have  disappeared,  the  arteries  of  the  retina 
are  small  and  the  veins  enlarged. 

Treatment  is  unavailing.  Eefer  the  patient  to 
an  oculist. 

As  all  of  the  ocular  diseases  here  described  are 
associated  with  disturbance  of  vision,  either  as  the 
cause  or  the  result  of  the  disease,  the  patient 
usually  consults  the  j-efractionist  first,  with  the 
idea  that  there  is  something  wrong  with  his  re- 
fraction, and  it  is  therefore  important  that  the 
rcfractionist  should  be  able  to  recognize  the  con- 
ditions and  acquaint  the  patient  with  the  true 
nature  of  his  trouble,  so  that  lie  may  secure  the 


DISEASES    OF    THE    EYE  181) 

hid  of  a  competent  oculist  witliout  delay — for  de- 
lays are  serious  in  all  eye  diseases.  A  careful  at- 
tention to  the  characteristic  symptoms  and  ap- 
pearances above  set  forth  will  very  quickly  enable 
any  refraction ist  of  ordinary  intelligence  and  ob- 
servation, who  is  on  the  lookout  for  possible  patho- 
logical lesions,  to  detect  and  identify  them. 


CHAPTER  XVI. 

FITTING  THE  GLASSES. 

Principles  Involved. 

It  is  highly  important  that,  after  being  cor- 
rectly prescribed,  the  glasses  be  properly  fitted 
to  the  patient's  eyes  and  face,  and  the  refrac- 
iionist  should  attend  to  this  feature  of  the  treat- 
ment, either  by  adjusting  the  frames  or  mount- 
ings himself,  if  he  be  a  practical  optician,  or  by 
carefully  prescribing  every  detail  for  the  opti- 
cian's guidance.  Tabulated  forms  usually  accom- 
pany the  prescription  blanks  furnished  by  opti- 
cians for  the  proper  measurement  of  frames  and 
mountings,  and  these  should  be  fully  and  care- 
fully followed  out  in  prescribing. 

These  mechanical  details  and  the  various  meth- 
ods that  have  been  devised  for  carrying  them  out 
are  all  designed  to  realize  a  few  definite  optical 
principles.  The  first  and  probably  most  important 
of  these  principles  is!  that  which  pertains  to  the 
relation  between  the  visual  axes  of  the  eyes  and 
the  optical  centers  of  the  lenses,  and  may  be  ex- 
pressed as  follows: 

The  visual  axis  of  the  eye  should  cut  the  mean 
plane  of  the  lens  'perpendicularly ,  at  its  optical 
center. 

This  principle  involves  two  distinct  condition.^. 
First,  the  optical  center  of  the  lens  must  coincide 


192  REFRACTION 

with  the  visual  axis,  and,  second,  the  plane  of  the 
lens  must  stand  at  right  angles  to  the  visual  axis. 

Pupillary  Distance. — The  adjustment  of  the 
lenses  so  that  their  optical  centers  coincide  with 
the  visual  axes  is  attained  by  measuring  the  dis- 
tance between  the  centers  of  the  pupils  and  mak- 
ing the  distance  between  the  centers  of  the  lenses 
correspond  with  it.  This  measurement  can  bo 
made  with  an  approximate  degree  of  accuracy  bv 
having  the  patient  look  steadily  at  the  observer's 
forehead  and  applying  a  properly  graduated  foot- 
rule  across  the  bridge  of  his  nose.  In  performing 
this  maneuver,  however,  it  is  essential  that  the 
rule  be  held  as  close  as  possible  to  the  patient's 
eyes  and  as  far  as  possible  from  the  observer's, 
else  the  distance  as  seen  upon  the  rule  will  in 
reality  fall  within  the  pupillary  centers,  and  the 
resulting  measurement  will  be  too  little. 

Dr.  Maddox'  Pupil  Localizer. — For  more  re- 
liable measurement  some  mechanical  devices  have 
been  constructed,  of  which  the  simplest  is  Dr. 
Maddox'  pupil  localizer,  a.  little  instrument  rep- 
resented in  the  accompanying  cut.  One  pupil  is 
located  at  a  time  with  this  instrument,  its  prin- 
ciple being  similar  to  that  of  the  "sighter"  on  a 
common  shotgun.  It  is  placed  on  the  ordinary 
trial  frame  and  brought  in  front  of  the  patient's 
eyes  so  that  the  observer's  eye  sights  the  trian- 
gular projection  immediately  in  the  center  of  the 
patient's  pupil.  The  graduated  bar  on  the  trial 
frame  then  shows  how  far  from  the  middle  the 


FITTIXO    THE    GLASSES 


193 


eye-center  is.  The  operation  is  repeated  on  tin; 
other  side,  and  the  sum  of  the  two  distances  fro:n 
the  middle  is  the  pupillary  distance.  One  of  the 
advantages  of  this  method  is  that  it  discloses  any 
lack  of  symmetry  there  may  he  in  the  respective 
distances  of  the  pupils  from  the  median  line. 

Generally,  however,  a  sufficiently  accurate  meas- 
urement of  the  pupillary  distance  is  obtained  from 


lUustrates  graduated  trial  frame,  by  which  pupillary 
distance,  temple  measurements,  and  height  of  bridge 
can   be  estimated. 

the  graduated  bar  of  the  trial  frame  without  any 
special  device,  especiallv  if  the  pin-hole  disc  has 
been  used.  Most  of  the  trial  frames  now  sup- 
plied are  furnished  with  an  indicator  which  de- 
notes the  pupillary  distance  represented  by  the 
width  of  the  frame  at  which  the  refractionist  has 
worked. 

Perpendiculakity  of  the  I'lane. — It  will 
readily  be  seen  that  a  lens  exerts  its  proper  diop- 
tric strength  only  when  its  plane  is  perpendicular 
to  the  visual  axis;  for  if  the  visual  axis  passes 
through  it  obliquely,  then   tlie  lens  has  a  greater 


194 


REFRACTION 


convexity  or  concavity,  as  the  case  may  be,  toward 
the  eye.  If  it  be  a  cylindrical  lens,  it  simply  acts 
as  a  still  stronger  cylinder,  if  it  be  a  spherical 
lens,  it  not  only  acts  as  a  strongei*  sphere,  but 
takes  on  the  nature  and  effect  of  an  added  cylin- 


Illustrates  the  perpendicular  plane  of  the  lenses  for 
near  and  distant   vision,  respectively. 

der  having  its  axis   at  right  angles   to  the  axis 
about  which  the  ]ens  is  rotated. 

With  lenses  of  weak  dioptrism,  i.  e.,  of  slight 
curvature,  this  feature  is  of  course  relatively  un- 
important, as  the  increase  of  effect  is  negligible; 
but  the  stronger  lens  is,  the  more  noticeable  it 


FITTING   THE   GLASSES  195 

becomes,  and  with  ver}'  strong  lenses  it  is  extreme- 
ly important. 

It  is  quite  apparent  that  an  ideal  condition  of 
perpendicularity  is  impossible  of  attainment,  for 
the  lens  can  only  be  placed  so  as  to  be  perpen- 
dicular to  the  visual  axis  when  the  eye  is  in  one 
particular  position,  and  any  deviation  of  the  eye 
from  that  position  will  disturb  the  relation.  For 
example,  if  the  glasses  are  so  adjusted  that  the 
visual  axes  of  the  eyes  at  rest,  i.  e.,  parallel,  cut 
the  plane  of  the  lenses  at  right  angles,  then  the 
moment  the  eyes  are  converged  the  visual  axes 
will  cut  the  plane  obliquely,  and  vice  versa. 

The  best  that  can  be  done,  therefore,  is  to  ad- 
just the  glasses  so  that  the  visual  axes  cut  the 
plane  perpendicularly  when  the  eyes  are  in  that 
position  which  most  nearly  corresponds  with  the 
use  for  which  the  glasses  are  commonly  used. 

If  the  glasses  are  to  be  used  for  distant  vision 
only,  then  the  visual  axes  are  taken'  as  parallel, 
and  the  lenses  are  adjusted  in  one  plane,  a  verti- 
cal plane,  at  right  angles  to  the  axes.  When  the 
glasses  arc  prescribed  for  near  work,  such  as 
reading,  sewing,  etc.,  the  inclination  of  the  visual 
axes  is  calculated  from  the  degree  of  convergence 
proper  to  the  distance  at  which  the  work  is  to 
be  (lone,  and  the  lenses  adjusted  in  two  opposite 
planes,  inclined  to  each  other,  each  at  right  angles 
to  its  visual  axis.  In  cases  where  the  glasses  are 
intended  for  constant  wear,  the  general  rule  is  to 
adjust  the  lenses  at  right  angles  to  visual  axes  at 


]9G 


REFRACTION 


an  inclination  midway  between  the  parallel  and 
the  convergence  proper  to  the  patient's  working 
distance. 

DowNWAKD  Tilting. — In  addition  to  the  in- 
clination of  tlie  planes  inward  for  near  vision,  it 


lUustrates    the    Perpendicular    Axis    Effect    of    Peri- 
scopic  Lens. 


is  also  necessary  that  they  incline  slightly  down- 
ward, so  as  to  provide  for  the  lowering  of  the  eyes. 

Bi-FOCALS. — In  adjusting  bi-focals,  the  planes 
of  the  lenses  should  be  set  between  that  of  dii- 
tant  and  near  vision,  but  should  incline  a  little 
more  toward  the  perpendicular  of  the  stronger 
lens. 

Periscopic  Glasses. — We  have  already  seen 
that  this  adjustment  of  the  lens  perpendicularly 
to  the  visual  axis  is  only  possible  in  one  position 
of  the  eye,  and  as  soon  as  the  patient  turns  his  eye 


FITTING    TllK    (il.ASSKS 


197 


SO  as  to  look  through  the  outer  peripheries  of  his 
glasses,  his  visual  axes  cut  the  lens  obliquely,  giv- 
ing the  effect  of  stronger  lenses.     With  a  view  of 
obviating  this  trouble  as  mucli  as  possible,  par- 
ticularly in  the   case  of  strong  lenses,  the  peri- 
scopic  or  toric  form  of  lenses  have  been  devised. 
These  arc  concavo-convex  lenses,  such  as  have  al- 
ready been  described  in  tlic  chapter  on  Lenses,  and 
are  made  so  that  their  inner  curvature  conforms 
as  nearly  as  practicable  to  the  sphere  of  rotation 
of  the  eyeball.     In  this  way  the  visual  axis,  no 
matter  which  way  the  eye  is  turned,  cuts  the  lens 
approximately  at  right  angles  to  the  mean  plane 
of  its  inner  cuvature,  and  while  the  outer  curva- 
ture is  not  entirely  coincident  with  the  inner,  yet 
the  oblique  effect  is  very  greatly  modified  by  this 
arrangement.     The  periscopic  lenses  are  therefore 
always  to  be  preferred  in  cases  of  strong  spherical 
correction. 

So  far  no  satisfactory  form  of  periscopic  cyl- 
inder lenses  has  been  devised,  although  Dr.  Geo. 
C.  Harlan  has  called  attention  to  a  method  of 
producing  a  toric  effect  with  crossed  cylinders  in 
compound  astigmatism,  by  grinding,  for  example, 
-|-4  D.  and  -f6  D.  on  one  side  of  the  glass  and 
—2  D.  on  the  other  side,  to  give  the  effect  of 
4-2  D.  and  +4  D.  combined  cylinders. 

Transposition. — In  further  pursuit  of  this 
purpose,  it  is  always  advisable,  in  prescril)ing  a 
combined  correction  which  comi)incs  a  high  plus 
sphere  with  a  high  phis  cvhnder,  or  a  high  minu^ 


198  REFR ACTION 

sphere  with  a  liigh,  minus  cylinder,  to  transpose 
tJie  cylinder  and  increase  the  sphere  correspond- 
ingly, so  as  to  get  more  or  less  of  a  toric  effect. 
For  example,  a  combination  of 

+  12  D.  spher.  +8  D.  cyl.  ax.  180°. 
should  be  transposed  to 

+20  D.  spher.  —8  D.  cyl.  ax.  90°. 
Prescribing  of  Glasses. 
The  carrying  out  of  the  optical  principles  above 
expounded  in  the  fitting  of  glasses  to  the  face 
and  eyes  is  provided  for  in  the  various  mechan- 
ical parts  of  the  frames  and  mountings,  and  in 
order  that  the  prescriber  may  intelligently  insure 
the  realization  of  these  principles  it  is  essential 
that  he  have  a  clear  understanding  of  the  nature 
and  effect  of  the  different  attachments  of  the 
glasses. 

CoMPOXENT  Parts. — Spectacles  and  eye-glassen 
are  made  either  with  or  without  rims,  or,  as  the 
optician  calls  them,  eye-wires.  In  the  rimmed 
variety,  a  pair  of  spectacle  frames  consists  of  the 
e3^e-wires,  end  pieces,  temples,  and  bridge.  The 
end  pieces,  four  in  number,  are  attached  in  pairs 
to  the  outer  sides  of  the  eye- wires;  one  of  the  end 
pieces  is  furnished  with  a  projecting  pin  or  dowel, 
which  fits  into  a  receiving  hole  in  the  opposite 
piece.  Both  pieces  are  pierced  with  a  threaded 
hole  for  a  screw.  The  temple  is  flattened  at  its 
near  end  and  pierced  with  a  hole  corresponding  to 
the  pin  on  the  end-piece.  The  temple  is  placed 
between    the    two    end-pieces,    the    pin    passing 


FITTING    THE    GLASSES 


199 


through  the  hole  in  the  temple  into  the  reception 
hole  in  the  opposing  end-piece.  A  screw  is  then 
driven  throngh  the  threaded  holes  in  the  end- 
pieces,  so  as  to  hold  the  lens  in  place  in  the  tight- 
ened  eye-wire   and    tlie  temple  between  the  end- 


END  PIECE 


END  PIEC6 


Illustrates  the   component  parts   of  spectacles. 


pieces.  The  same  process  is  repeated  on  the  other 
temple.  The  temples  revolve  on  the  pin  inward, 
but  are  prevented  from  revolving  outward  by  a 
small  flange  on  the  temple.  The  bridge  is  soldered 
to  the  inner  sides  of  the  eye-wires. 

In  rimles9  spectacles  the  general  arrangement 
is  the  same,  except  tliat  the  end-pieces  and  the 
bridge  are   attached   to   the  lenses  themselves  by 


200 


REFRACTION 


means  of  small  clutches  which  are  screwed  onto 
the  lenses  through  holes  drilled  in  the  glass. 

In  the  eye-glasses^  of  course,  there  are  no  tem- 
ples.    On  the  right  outer  side  of  the  eye-wire  (or 


HOLE  FOR  CORD 

Illustrates   the    component   parts   of   eye-glasses. 

lens  in  rimless  eye-glasses)  the  end-piece  is  en- 
larged into  a  post  and  handle  with  which  to  ma- 
nipulate the  glasses.  In  place  of  the  bridge  there 
are  attached  to  the  inner  sides  of  the  eye-wires 
or  lenses  two  studs,  into  which  are  inserted  the 
lower  ends  of  the  spring.  To  these  inner  sides 
of  the  glasses  are  also  attached  the  nose-pieces, 
which  are  usually  furnished  with  a  guard  of  some 
resilient  material  to  keep  them  from  abrading 
tlio  nose.     Very  crude  eye-glasses  lack  the  nose- 


FITTING   THE   GLASSES 


201 


piece^  the  eye-wire  or  the  lens  itself  clasping  the 
nose^  but  such  articles  are  nowadays  extremely 
lare. 

Temples. — Fundamentally  the  only  two  vari- 
eties of  temple  are  those  which  curl  around  the 
back  of  the  ear  and  those  which  do  not,  the  only 
practical  difference  between  them  being  one  of 
comfort.  The  former,  "hook"  or  "ridingi  bow," 
are  better  for  constant  wear,  the  latter  or 
•'straight-'  for  patients  who  have  to  be  continually 
putting  them  on  and  taking  them  off.  Nowadays 
some  of  the  riding  bows  are  made  of  flexible  metnl 


Illustrates  the  different   kinds   of  V)i-idge 


and  are  known  as  "cable  riding  bows"  in  distinc- 
tion from  the  firm  variety  whicli  are  called 
"curled." 

Bridges. — As  the  bridge  is  tlic  part  whose  shape 
and  position  regulate  the  lateral  and  vertical  re- 
lation of  the  lenses  to  the  eyes,  its  adjustment 
is  a  matter  of  great  imnortance  and  demands  care- 
ful attention.     The  width  of  the  bridge  (the  lat- 


202  REFRACTION 

eral  diameter  of  tlie  lenses  being  known)  deter- 
mines the  distance  between  the  centers  of  the 
lenses,  and  hence  the  coincidence  of  the  lenses 
with  interpupillary  distance.  The  arch  of  the 
bridge  and  the  point  of  its  attachment  to  the 
glasses  determine  the  centering  or  decentering  of 
the  lenses  up  or  down. 

Bridge  Measurements. — There  are  inniimei-- 
able  varieties  of  bridges,  which  it  would  be  folly  to 
attempt  to  enumerate,  far  less  describe,  here.  They 
may  be  found  in  the  catalogues  of  the  various 
optical  firms.  They  fall,  however,  like  the  tem- 
ples, into  two  fundamental  classes,  the  wire  and 
the  saddle  bridge.  The  former  are  intended  to  be 
shaped  to  the  nose  and  are  all  right  for  persons 
having  good  noses,  but  for  the  average  nose  the 
saddle  bridge,  made  in  the  shape  of  a  letter  K, 
which  may  be  as  flat  or  as  deep  as  required,  serve 
the  purpose  best. 

The  height  of  the  bridge  is  the  distance  from 
a  line  joining  the  centers  of  the  lenses  to  the  top 
or  crest  of  the  bridge.  Hence,  if  we  hold  a  rule, 
or  other  straight  edge,  across  the  patient^s  pupils, 
resting  on  the  bridge  of  the  nose,  the  height  of 
this  edge  above  the  pupils  will  give  the  necessary 
height  of  the  crest  of  the  bridge.  In  some  cases 
it  will  be  found  that  the  crest  of  the  bridge  must 
be  below  the  centers.  This  height  of  the  bridge 
top  is  adjusted  by  raising  or  lowering  the  arch 
of  the  bridge,  which  is  always  attached  at  the 
lateral  centers  of  the  lenses. 


FITTING    THE    (iLASSES         '  203 


''.IJie  adjusliiiL'iit  of  the  bridj'o  in  relation  to 
the  lenses  also  determines  the  relation  which  the 
plane  of  the  lens  will  bear  to  the  visual  axis,  the 
lenses  being  made  to  incline  inward  or  outward, 
upward  or  downward,  by  the  ana'le  at  which  the 
bridge  is  attached  to  the  glasses.  It  is  also  by 
this  means  that  the  proper  distance  of  the  lenses 
from  the  eye  is  regulated.  In  shallow  noses  the 
crest  of  tlie  bridge  needs  to  bo  considerably  back 
of  the  lenses  so  as  to  push  them  forward;  in 
prominent  noses  and  deep-set  eyes  the  reverse. 

In  order  that  the  wire  bridge,  when  used,  may 
properly  fit  the  nose,  it  is  necessary  in  prescribing 
to  give  the  width  of  the  nose  at  the  place  where 
the  base  of  the  bridge  will  rest. 

Eye-Glasses. — In  the  eye-glass  the  bridge  is 
replaced  by  the  spring  and  the  nose-piece.  Like 
the  bridges  of  spectacles,  their  variety  is  legion, 
and  they  obey  the  same  principles  as  the  bridge, 
naniel}^,  in  determining  the  position,  plane,  and 
distance  from  the  eyes  of  the  lenses,  according 
to  the  different  angles  at  which  the  guards  are 
attached  to  the  lenses,  and  the  distance  above  or 
below  the  center  at  which  the  studs  are  attached 
to  the  lenses. 

Formerly  the  great  objection  to  eye-glasses  was 
the  tendency  of  the  spring  to  turn  the  glasses 
askew  and  so  displace  the  axis  of  a  cylinder  or  the 
base  of  a  prism,  but  modem  ingenuity  has  almost 
entirely  overcome  this  objection  except  in  the  case 
of  very  inappropriate  faces  and  noses. 


204  REFRACTION 

The  Studs. — In  eye-glasses,  as  in  spectacles,  it 
is  often  necessary  to  set  the  glasses  forward  or 
backward,  according  to  the  prominence  or  shal- 
lowness of  the  eyes.  This  is  done  by  means  of 
special  studs,  known  as  inset  and  outset  studs. 

The  pupillary  distance  in  eye-glasses  is  regu- 
lated by  the  size  of  the  lenses  in  connection  with 
the  measurement  of  the  nose  itself,  and  in  pre- 
scribing eye-glasses,  therefore,  it  is  always  nec- 
essary to  give  the  width  of  the  patient's  nose  at 
tlie  point  where  the  top  of  the  guards  will  come, 
and  where  the  bottom  of  the  guards  will  be. 

Size  of  Eye. — The  size  of  the  lens  should  al- 
ways be  sufficient  to  enable  the  patient  to  com- 
pass an  ordinary  range  cf  rotary  vision  without 
looking  over  or  under  or  to  the  side  of  his  glasses. 
The  lenses  are  cut  in  different  grades  of  size,  and 
numbered,  of  which  No.  00  is  the  largest  ordinary 
size  and  N"o.  5  the  smallest.  Larger  or  smaller 
than  this  are  designated  by  adding  an  0  or  in- 
creasing the  numeral  respectivel3^ 

Bi-i'OCALS. — These  are  designed  td  obviate  tho 
trouble  of  changing  glasses  by  patients  who  are 
obliged  to  wear  different  power  lenses  for  distant 
and  near  vision.  The  lower  part  of  the  lens  is 
made  to  differ  in  dioptric  power  from  the  upper 
part.  These  glasses  are  sometimes  called  Frank- 
lin glasses,  because  Franklin  invented  them. 
There  are  various  methods  of  constructing  th?m. 

Ground  hi-focals  are  made  from  one  piece  of 
glass,  which  is  sfround  to  one  dioptrism  on  the 


FITTING   THE   GLASSES 


205 


upper  surface  and  to  another  on  tlie  lower.  Ai 
it  is  impossible  to  center  both  spherical  surfac?s 
upon    the    same    piece    of   'glass,    liowover^   these 


Illustrates  the  different  styles  of  bi-focals. 

glasses  always  produce  a  prismatic  effect,  and  are 
therefore  not  in  common  use. 

Cemented  hi-focals  are  made  by  grinding  the 
whole  lens  of  the  same  dioptrism,  and  then  ce- 
mentinfr  to   the  lower  sni-face  another  thin  lens. 


206  REFRACTION 

known  as  a  wafer,  ground  to  the  required  addi- 
tional dioptrism.  These  bi-focals  are  light,  strong, 
and  reac  ly  permit  of  changes  in  the  near  cor- 
rection. In  prescribing  these  bi-focals  the  refrac- 
tionist  should  be  careful  to  indicate  the  degree  of 
inset  decentration  he  desires  for  the  reading  wa- 
fers. 

Achromatic  M-focah,  which  are  used  in  apha- 
kia, and  are  designed  to  do  away  with  chromatic 
aberration,  are  made  by  grinding  the  distant  di- 
optrism in  the  form  of  two  piano-spheres,  each 
being  scooped  out  in  its  lower  surface  and  the 
supplemental  lens  inserted  between  them.  The 
distance  lens  is  made  of  crown  glass  and  the  sup- 
plemental of  flint,  the  difference  of  refractive  in- 
dex obviating  chromatic  aberration. 

Half-lenses. — For  persons  who  need  only 
lenses  for  near  vision  glasses  are  now  made  con- 
sisting onlv  of  the  lower  halves  of  the  lenses, 
through  which  the  person  looks  down  when  read- 
ing, etc.,  and  looks  ever  them  when  employing  his 
distant  vision.  Similar  glasses,  consisting  only  of 
the  upper  halves,  are  made  for  myopic  persons 
who  need  no  lens  for  near  vision. 

DecentePiING  of  Lenses. — As  we  have  already 
seen,  a  lens  is  said  to  be  centered  when  its  op- 
tical center  (i.  e.,  the  center  of  its  curvature) 
coincides  with  the  visual  axis  of  the  eye  in  a 
state  of  rest;  and  as  the  visual  axis  of  the  eye 
in  a  state  of  rest  is  supposed  to  pass  through  the 
geometric  center  of  the  lens  (i.  e.,  the  point  which 


FITTING    THE    GLASSES 


207 


is  equidistant,  vertically  and  laterally,  from  the 
circumference)  a  lens  is  regarded  as  being  cen- 
tered when  lis  geometric  and  optical  centers  are 
Identical.  When  this  is  not  the  case,  i.  e.,  when 
the  optical  center  is  to  one  side  or  the  other  of 


Illustrates    a    centered    lens,    in    which    the    optical 
center  O  and  the  geonnetric  center  G  coincide. 


the  geometric  center,  the  lens  is  said  to  be  de- 
centered.  When  the  optical  center  is  to  the  inner 
side  of  the  geometric  center  it  is  said  to  be  "de- 
centered  in";  when  the  reverse,  it  is  "decentered 

out." 

Prtsmatic  Effect  of  Decentering. — Tt  can 
readily  he  understood  that  the  effect  of  a  spherical 


208 


REFRACTION 


lens  which  has  been  decentered  is  that  of  a  prism, 
because  at  any  point  in  a  spherical  lens  other  than 
its  optical  center  the  planes  of  the  two  surfaces 


Illustrates  the  prismatic  effect  of  decentering.  The 
optical  center  O  has  been  moved  toward  the  base  of 
the  vertical  prism  BAG. 

of  the  lens  are  not  parallel,,  but  are  inclined  to 
each  other  at  an  angle  whose  degree  depends  upon 
the  degree  of  curvature. 

When,  therefore,  tlie  visual  axis  passes  through 
the  lens  at  any  other  point  than  its  optical  center 
the  effect  is  the  same  as  looking  tlirough  a  prism. 
\\lietber  the  prism  so  produced  has  its  base  in  or 
out,  up  or  down,  depends  upon  Avhether  the  sphere 


FITTING   THE   GLASSES  ,    201) 

is  convex  or  concave,  and  upon  the  direction  in 
winch  it  is  decentered.  Since  a  convex  lens  is 
thickest  at  its  optical  center,  the  base  of  the  prism 
produced  by  its  decentering  always  looks  toward 
the  optical  center  of  the  lens;  hence  when  a  con- 
vex lens  is  "decentered  in/'  the  prismatic  effecL 
is  that  of  base  in,  when  "decentered  out"  it  is  tbat 
of  base  out.  A  concave  lens  being  thinnest  at  its 
optical  center,  the  prismatic  effect  of  decentering 
always  has  its  base  looking  away  from  the  optical 
center;  hence  when  a  concave  lens  is  "decentered 
in''  it  gives  a  prismatic  effect  base  out,  and  vice 
versa. 

Prescribing  Decentered  Lenses. — As  a  de- 
centered  spherical  lens  has  the  effect  of  a  spherical 
lens  of  the  strength  indicated,  plus  a  prism  of  the 
character  and  strength  corresponding  to  the  de- 
gree of  concentration,  and  as  it  is  often  desirable 
to  combine  a  prism  with  a  sphere,  lenses  are  not 
infrequently  prescribed  to  be  decentered  so  as  to 
give  the  required  prismatic  effect. 

The  stronger  the  lens,  of  course,  the  smaller  the 
degree  of  decentration  necessary  to  produce  the 
same  prismatic  effect.  The  degree  of  prismatic 
effect  produced"  by  decentering  a  lens  of  1  D.  to  the 
extent  of  1  mm.  is  called  the  prismdioptre  or  cen- 
trad.  Hence  in  determining  the  amount  of  de- 
centration necessarv  to  produce  a  given  number 
of  centrads  with  n  lens  of  n  given  streugth,  we 
simply  divide  tbe  number  of  cenlrads  required 
by  the  dioptrism  of  the  lens  nnd  iho  rpiotient  is 


21U  REFRACTION 

the  number  of  centimeters  of  decentration  neces- 
sary. For  instance,  if  it  is  desired  to  produce  2 
centrads  with  a  lens  of  4  D.,  dividing  4  into  2 
gives  5;  the  necessary  decentration  is  .5  cm.,  or 
5  mm. 

If  the  refractionist  prefers  to  estimate  the  de- 
gree of  prismatic  effect  in  terins  of  the  prismatic 
angle,  then  he  has  only  to  remember  that  a  lens 
of  1  D.  must  be  decentered  9.4  mm.  in  order  to 
produce  a  prismatic  angle  of  1  degree.  Thus,  in 
determining  the  amount  of  decentration  neces- 
sar}',  lie  multiplies  9.4  by  the  prismatic  angle  he 
wishes  to  produce,  and  divides  by  the  dioptrism  of 
the  lens.  For  example,  with  a  lens  of  4  D.  he 
wishes  to  produce  2  deg.  prismatic  angle;  9.4  mul- 
tiplied by  2  is  18.8,  which  divided  by  4  gives  4.7. 
The  result  in  this  case  is  in  millimeters.  The 
lens  would  need  decentering  4.7  mm. 

In  prescribing  this  decentration  of  lenses  the 
I'cfractionist  may  either  indicate  the  prismatic 
angle  or  centrad  he  desires,  or  the  number  of 
millimeters  he  wishes  the  lens  decentered. 

Methods  of  Decentering. — Practically,  the 
decentering  may  be  accomplished  either  by  cutting 
the  lens  (after  it  is  ground)  so  that  the  optical 
center  falls  to  one  side  of  the  geometric  center,  or 
by  attaching  the  finished  lenses  to  the  bridge  oi' 
mounting  so  that  the  center  is  displaced  from  the 
normal  visual  axis.  The  former  method  is  pref- 
erable except  in  cases  where  the  strength  of  tlic 
lrn>   niul   the   degree   of   decentration   required   is 


FITTING   THE   GLASSES  211 

SO  great  as  to  increase  the  weight  to  an  incon- 
venient extent.  This  method  oilers  a  hirger  range 
of  usefulness  in  vertical  than  in  lateral  decentra- 
tion. 

Decentering  Equivalents.^ — The  following  is 
a  table  of  decentering  equivalents,  showing  the 
amount  of  decentration  in  millimeters  necessary 
to  produce  various  prismatic  angles  with  lenses 
of  (litferent  dioptric  strength. 


Lens 

1*^ 

2° 

3° 

4° 

5° 

6° 

8° 

10° 

1  D, 

9.4 

18.8 

28.3 

37.7 

47.2 

56.5 

75.8 

95.2 

2 

4.7 

9.4 

14.1 

18.8 

23.6 

28.2 

37.9 

47.6 

3 

3.1 

6.3 

9.4 

12.6 

15.7 

18.8 

25.4 

31.7 

I.ens 

1° 

2° 

3° 

4° 

5° 

6° 

8° 

10° 

4 

2.3 

4.7 

7.1 

9.4 

11.8 

14.1 

18.9 

23.8 

5 

1.9 

3.8 

5.7 

7.5 

9.4 

11.3 

15.2 

19. 

6 

1.6 

3.1 

4.7 

6.3 

7.9 

9.5 

12.6 

15.9 

7 

1.3 

2.7 

4. 

5.4 

6.7 

8.1 

10.8 

13.5 

8 

1.2 

2.3 

3.5 

4.7 

5.9 

7.1 

9.5 

11.9 

9 

1. 

2.1 

3.1 

4.2 

5.2 

6.3 

8.4 

10.5 

10 

.9 

1.9 

2.8 

3.8 

4.7 

5.6 

7.6 

9.5 

11 

.9 

1.7 

2.6 

3.5 

4.3 

5.1 

6.9 

8.7 

12 

.8 

1.6 

2.4 

3.1 

3.9 

4.7 

6.3 

7.9 

13 

.7 

1.4 

2.2 

2.9 

3.6 

4.3 

5.8 

7.3 

14 

.7 

1.3 

2. 

2.7 

3.4 

4. 

5.4 

6.8 

15 

.6 

1.3 

-1.9 

2.5 

3.1 

3.8 

5.1 

6.3 

ir, 

.6 

1.2 

1.8 

2.4 

3. 

3.5 

4.7 

6. 

17 

.6 

1.1 

1.7 

2.2 

2.8 

3.4 

4.5 

5.6 

18 

.5 

1. 

1.6 

2.1 

2.6 

3.1 

4.2 

5.3 

19 

.5 

1. 

1.5 

2. 

2.5 

3. 

4. 

5. 

20 

.5 

.9 

1.4 

1.9 

2.4 

2.8 

3.8 

4.8 

Below  is  a  similar  table,  but  with  the  degree  of 
prismatic  effect  expressed  in  centrads  or  prism- 
dioptres: 


Lens  1  Cr. 

2  Cr. 

3  Cr. 

4  Cr. 

5  Cr. 

6  Cr. 

S  Cr. 

10  cr. 

1  D,  10. 

20. 

30. 

40. 

50. 

60. 

80. 

100. 

2    5. 

10. 

15. 

20. 

25. 

30. 

40. 

50. 

3    3.3 

6.6 

10. 

13.3 

16.6 

20. 

26.6 

33.3 

4    2.5 

5. 

7.5 

10. 

12.2 

15. 

20. 

25. 

212 

EEFEACTION 

5 

o 

4. 

6. 

8. 

10. 

12. 

16. 

20. 

6 

1.6 

3.3 

5. 

6.6 

8.3 

10. 

13.3 

16.6 

7 

1.4 

2.8 

4.2 

5.7 

7.1 

8.2 

11.4 

14.2 

8 

1.2 

2.5 

3.7 

5. 

6.2 

7.5 

10. 

12.5 

9 

1.1 

2.2 

3.3 

4.4 

5.5 

6.6 

8.8 

11.1 

10 

1. 

2. 

3. 

4. 

5. 

6. 

8. 

10.9 

11 

.9 

1.9 

2.8 

3.7 

4.6 

5.5 

7.3 

9. 

12 

.8 

1.8 

2.5 

3.3 

4.1 

5. 

6.6 

8.3 

13 

.7 

1.5 

2.3 

3. 

•3.8 

4.6 

6.1 

7.6 

14 

.7 

1.4 

2.1 

2.8 

3.5 

4.2  . 

5.7 

7.1 

15 

.6 

1.3 

2. 

2.6 

3.3 

4. 

5.3 

6.6 

16 

.6 

1.2 

1.8 

2.3 

3.1 

3.7 

5. 

6.2 

17 

.5 

1.1 

1.7 

2.3 

2.9 

3.5 

4.7 

5.8 

18 

.5 

1.1 

1.6 

2.2 

2.7 

3.3 

4.4 

5.5 

19 

.5 

1.5 

2.1 

2.6 

3.1 

4.2 

5.2 

20 

.5 

1.5 

2. 

2.5 

3. 

4. 

5. 

Decentering  of  Eeading  Glasses.  —  All 
glasses  that  are  prescribed  for  reading,  sewing', 
and  other  forms  of  near  work  should  be  decen- 
tered,  in  connection  with  the  inclination  of  their 
planes  already  referred  to,  to  the  extent  demanded 
by  the  inclination  of  the  visual  axes  when  con- 
verged for  the  distance  worked  at.  This,  of  course, 
is  not  for  the  purpose  of  producing  a  prismatic 
effect,  but  to  prevent  it,  by  permitting  the  visual 
axis  to  pass  through  the  optical  center  of  the  lens. 
Glasses  for  constant  use  should  be  decentered  to 
an  extent  half  wav  between  the  geometric  center 
and  the  degree  required  for  near  work. 

Distance  of  Lens  from  Eye. — As  a  general 
proposition,  tlie  le^is  should  be  placed  as  near  to 
the  eye  as  possible  without  touching  the  eyelashes. 
In  estimating  the  refraction  of  the  eye,  the  con- 
vergence or  divergence  of  the  rays  as  they  enter 
or  emerge  from  the  eye  are  made  the  basis  of  cal- 
culation, hence  the  distance  between  the  correcting 


FITTING   THE   GLASSES  213 

k'lis  and  l.lie  surface  of  the  eve  should  be  as  near 
zero  as  possible.  In  cases  of  presbyopia,  however, 
and  other  instances  where  the  purpose  of  the 
glasses  is  to  adjust  the  accommodation  for  some 
particular  Morkiug-poiiit,  tlie  paiicjit  may  wear 
the  glasses  close  to,  or  at  a  distance  from,  the 
eyes,  according  to  the  distance  at  which  he  is 
working.  A  convex  lens  gains  in  elfect  the  fur- 
ther away  it  is  from  the  eye,  hence  the  further 
away  tlie  patient  holds  his  reading,  or  sewing,  or 
v/hatnoi:,  the  nearer  he  requires  the  glasses  to  be 
to  his  eyes,  and  vice  versa 


CHAPTER  XVII. 
HYGIENE  OF  THE  EYE. 

I'roiii  what  has  bmi  said  on  tlio  subject  of  eye- 
strain it  is  apparent  t!iat  (lie  hygiene  of  the  eye, 
so  far  as  its  visual  function  is  concerned,  falls 
into  two  general  divisions,  namely,  (1)  that  which 
pertains  to  the  muscular  elements,  accommoda- 
tion and  convergence,  and  (2)  tliat  which  has  to 
do  with  the  retina,  to  which  may  be  added  (3) 
that  which  concerns  the  ordinary  care  of  the  eye 

itself. 

Cr.osE    ArrLiCATiON. — Anyone    who    lias    ever 
plaved  the  schoolboy  trick  of  seeing  how  long  he 
conld  keep  his  arm  stretched  out  in  a  horizontal 
position  has  had  personal  experience  of  the  fa- 
tigue which  comes  from  maintaining  a  muscle  of 
the  body  in  a  continnons  state  of  even  partial  con- 
traction for  only  a  moderate  lengtli  of  time.     In 
the  case  of  the  experiment  referred  to  the  mind 
is  of  course  concentrated  upon  the  test  itself,  and 
the   fatigue  soon  becomes  so  noticeable  that  the 
experimenter   is   obliged   to   give   it   up.      In   in- 
stances, however,  where  the  continued  contraction 
IS   maintained   for  the   purpose   of   carrying   out 
some  muscular  work  upon  which  the  attention  is 
fixed,  the  sense  of  fatigue  does  not  manifest  itself 
until  a  rest  is  taken  or  the  muscle  gives  out  and 
refuses  to  contract  anv  longer. 

This  last  is  precisely  what  takes  place  in  the 
ciliary  and  internal  recti  muscles  of  the  eye  when 


216  REFRACTION  • 

the  vision  is  coucentrated  for  any  considerable 
length  of  time  upon  a  near  object.  These  mus- 
cles are  kept  in  a  continuous  state  of  quite  pow- 
erful contraction — the  slightest  let-up  would  in- 
terfere with  the  vision — but  the  mind  being  con- 
centrated upon  the  book  one  is  reading  or  the  work 
one  is  doings  the  terrible  exhausting  effects  do  not 
make  themselves  felt  until  either  the  muscles  are 
released  or  they  give  way  and  fall  into  little 
spasms,  in  which  event  the  vision  becomes  blurred 
of  course.  The  nearer  the  point  for  w^hich  the 
eyes  are  used  in  this  way,  the  greater  the  contrac- 
tion of  the  muscles  and  the  more  marked  the  fa- 
tigue. 

If  this  kind  of  abuse  is  persisted  in  (as  it  fre- 
quentlv  is),  the  ciliary  muscle — the  muscle  of 
accommodation — like  any  other  overworked  mus- 
cle of  the  body,  becomes  hypertrophied  and  the 
exercise  of  accommodation  becomes  to  that  extent 
a  constant  and  fixed  quantity.  In  this  way  a  lat- 
ent hyperopia  is  developed,  -which  requires  long 
and  patient  care,  even  after  proper  correction,  to 
remedy. 

In  addition,  the  continued  and  excessive  de- 
mand of  the  muscles  for  blood  during  their  pe- 
riod of  activity  induces  congestive  troubles  in  the 
eyeballs  and  lids. 

Frequent  Eests. — For  these  reasons  the  vision 
should  not  be  exercised  continuously  at  close  range 
for  any  great  length  of  time.  Persons  whose  oc- 
cupation   obliges   them    to   work   at   close   range 


HYGIENE    OF   THE   EYE  217 

should  make  a  practice  of  giving  their  eyes  fre- 
quent short  rests  by  removing  them  from  the 
work  and  completely  rehixing  ihcir  accommoda- 
tion and  convergence  for  a  few  minutes,  by  look- 
ing into  tlie  far  distance.  Those  wlio  do  excep- 
tionally close  and  exacting  work  for  very  long 
periods  at  a  stretch  should  relieve  their  accom- 
modation as  much  as  possible  by  the  use  of  ap- 
propriate convex  lenses  when  they  work,  and  their 
convergence  by  properly  adjusted  prisms. 

Children's  Eyes.— The  mischievous  results  of 
continued  maintenance  of  accommodation  and  con- 
vergence are  particularly  marked  in  young  chil- 
dren, whose  musculature  is  in  a  formative  stage. 
The 'vast  crop  of'  hyperopes,  squints,  and  asthe- 
nopes  that  throng  the  offices  of  the  refractionists 
at  later  ages  have  at  least  ninet^  per  cent,  of  then- 
origin  in  the  abuse  of  the  eyes  during  school  days. 
Fortunately  we  are  now  tardily  waking  up  to  tie 
dangers   of  the  case,  and  taking  steps  to  avoid 

them. 

In  the  first  place,  children  should  not  be  re- 
quired to  decipher  very  small  or  close  characters 
at  all.  Their  books  should  all  be  printed  in  mod- 
erately large  and  very  plain  type,  and  held  at  a 
respectable  distance  from  the  eye.  And  in  the 
second  place,  their  tasks  should  be  so  arranged 
as  to  o-ive  them  the  least  possible  amount  of  close 
work  and  the  greatest  possible  alternation  of  work 
requiring  no  effort  on  the  part  of  the  eyes. 

All  of  the  above  mentioned  evils  of  contmuea 


218  REFRACTION 

application  are  of  course  intensified  a  hundred- 
fold when  an}'  error  of  refraction  exists. 

Poor  Illumination. — Near-work  done  under 
the  handicap  of  poor  illumination  has  precisely 
the  same  effect  as  that  which  is  done  under  con- 
ditions of  excessive  exactingness.  The  inadequate 
intensity  of  the  image  made  upon  the  retina  hy 
the  insufficient  light  requires  that  the  image  he 
held  there  for  a  greater  length  of  time  in  order 
to  make  a  proper  impression  upon  the  brain,  and 
that  faculty  of  the  brain  which  represents  acuity 
of  vision  is  taxed  to  its  utmost  in  recognizing 
the  image.  Thus  the  musculature  and  the  central 
nervous  system  both  suffer.  The  question  of  what 
constitutes  adequate  illumination  will  be  discussed 
in  a  later  paragraph. 

Retinal  Hygiene. 

Excessive  Illumination. — Too  much  light  is 
almost,  if  not  quite,  as  bad  as  too  little.  It  must 
be  remembered  that  the  retina  is,  in  effect,  a  sen- 
sitive web  of  nerve  filaments,-  upon  which  light 
acts  as  an  irritant  just  as  the  teasing  of  a  cutane- 
ous ner\'e-end  causes  sensations  of  pain,  heat,  etc., 
as  the  case  may  be.  Now,  if  the  stimulation  of  a 
cutaneous  nerve-end  be  carried  too  far  the  pain, 
heat,  etc.,  become  injurious  both  to  the  nerve  and 
to  the  individual. 

The  same  thing  holds  good  in  the  case  of  the 
filaments  of  the  optic  nerve  spread  out  upon  the 
retina.     A  moderate  degree  of  irritation  by  light 


HYGIENE    OF    THE    EYE  219 

IS  necessary  to  the  production  of  the  sensation  of 
vision;  but  if  tlie  stimulation  be  too  vigorous  the 
nerve-ends  of  tlie  retina  quickl}^  become  inflamed 
or  exhausted,  and  rciiiiilis  or  retinal  asthenopia 
ensues.  i      -^ 

This  is  just  what  happens  wlicn  the  eye  is  con- 
stantly receiving  too  much  lights  whether  it  come 
direct  from  the  soui'ce  of  light,  as  by  continually 
facing  a  bright  window,  looking  into  glaring  fur- 
naces, etc.,  or  is  reflected  from  one's  work,  as  in 
reading  by  a  h^n-d  brilliant  light.  These  practices, 
therefore,  should  be  avoided  as  much  as  possible, 
and  if  one's  employment  is  such  that  they  are  un- 
avoidable, then  he  should  wear  smoked  glasses  to 
protect  the  retina. 

Guttering  Material  and  Colors. — A  very 
frequent  cause  of  this  tvpe  of  eye-strain  is  the 
excessive  reflection  of  light  by  the  object  upon 
which  one  is  engaged.  The  light  may,  in  itself, 
be  moderate  enough,  but  the  surface  of  the  work 
is  such  that  it  reflects  excessively.  Printers  and 
metal-workers  experience  this  kind  of  trouble. 
The  only  effective  prophylaxis  is  the  wearing  of 
tinted  glasses,  and  frequent  short  periods  of  rest. 

Those  who  are  obliged  to  concentrate  their 
vision  for  long  periods  upon  one  or  more  bright 
colors  suffer  not  only  from  general  retinal  ex- 
haustion, but  from  a  specific  exhaustion  of  those 
nerve  fibers  which  respond  to  the  color-vibrations 
in  question.  Their  only  means  of  relief  lies  in  the 
wearing  of  glasses  of  such  a  color  as  will  neu- 


220  REFRACTION 

tralize  the  offending  color  into  a  more  or  less 
mixed  and  quiet  tint.  Failing  this,  they  should 
frequently  rest  the  fatigued  color  nerves  by  gaz- 
ing steadily  for  a  few  minutes  at  the  color  which 
is  complementary  to  the  one  that  distresses  them. 

The  Eyeball. 

Dust. — A  very  common  source  of  trouble  to  the 
eyeball,  especially  in  large  cities  in  these  days  of 
busy  traffic,  is  found  in  the  clouds  of  dust  that 
circulate  in  the  air,  and  lodge  upon  the  conjunc- 
tiva. Those  who  wear  glasses  are  more  fortunate 
in  this  respect  than  their  fellows  whose  vision  does 
not  require  them  to  do  so,  for  the  lenses  serve  to 
keep  out  a  great  amount  of  dust. 

The  effect  of  this  continued  invasion  of  dust,  in 
greater  quantities  than  the  tear-ducts  and  winkers 
can  take  care  of,  is  frequently  to  set  up  a  mild 
chronic  conjunctivitis.  In  some  cases  it  induces 
a  still  more  chronic  process  of  productive  inflam- 
mation, by  which  a  web  of  connective  tissue  i^'rows 
over  the  sclera,  forming  what  is  known  as  a 
pterygium.  When  this  encroaches  upon  the  cornea 
it  gives  rise  to  astigmatism,  and  has  to  be  removed. 

The  only  effective  safeguard  against  the  inva- 
sion of  dust  consists,  of  course,  in  tlie  wearing  of 
goggles.  It  is  hardly  to  be  expected,  however,  that 
tlie  ordinary  man  will  submit  to  any  such  unes- 
thetic  adornment,  and  most  people  will  prefer  to 
take  their  chances  with  ordinary  precautions. 
Those  who  are  out  in  the  dust  a  great  deal  will 


HYGIENE    01-'   THE   EYE  221 

find  it  bcneiieiai  to  wash  their  eyes  each  evening, 
Avheu  they  get  lionie,  with  a  copious  instillation  of 
boric  acid  sohition  or  other  mild  astringent. 

Cold. — The  membrane  of  the  eye  is  subject  to 
the  same  ill  results  oi*  sudden  changes  of  tempera- 
ture as  other  membranes  of  the  body  having  simi- 
lar exposure.  As  a  rule  it  is  quite  able  to  adjust 
itself  to  moderate  variations,  such  as  one  encoun- 
ters in  daily  life.  But  extreme  variations,  es- 
pecially when  accompanied  by  high  winds,  will 
congest  the  conjunctiva  and  produce  a  troublesome 
conjunctivitis.  When  facing  a  more  than  ordi- 
narily severe  cold  or  heat  the  eve  should  be  kept 
closed  for  a  little  while  until  the  conjunctiva  be- 
comes gradually  warmed  or  cooled,  as  the  case 
may  be.  In  cases  of  very  extraordinary  exposures 
it  is,  of  conrse,  necessary  to  wear  goggles. 

Infection". — It  cnnnot  be  too  insistently  boi-no 
in  mind  that  the  conjunctiva  is  a  membrane  of 
very  rapid  and  ready  absorptive  capacity,  and  pre- 
sents a  most  facile  surface  for  the  entrance  of  in- 
fective matter  of  all  kinds.  The  most  criminal 
carelessness  is  displayed  in  this  respect  by  the 
average  person,  and  all  sorts  of  infection  are  con- 
veyed to  the  eve,  and  by  parents  to  their  chil- 
dren's eyes,  by  ]'ub1)ing  the  e3Tlid,  taking  out  for- 
eign objects,  and  otherwise  manipulating  the  eye 
with  fingers  carrying  infective  matter. 

Scrupulous  care  should  be  exercised  at  all  time.-: 
to  preserve  the  eye  from  this  type  of  injury. 
AAHienever  it  is  necessary  to  manipuhito  Iho  eye  or 


222  REFRACTION 

the  lids  for  an\-  reasoii_,  the  hands  should  be  cai'e- 
fully  washed;  if  a  haudkerchief  or  cloth  is  used 
it  should  be  scrupulously  cleau;  any  drops  or 
wash  that  is  instilled  should  be  prepared  in  sterile 
water,  or  at  least  clean  distilled  water;  and  after 
any  such  manipulation  the  eye  should  be  thor- 
oughly flushed  with  a  solution  of  warm  boric  acid 
or  other  mild  disinfectant. 

The  necessity  of  precaution  in  this  respect  is  es- 
pecially binding  upon  persons  who  are  suffering 
with  an  infective  trouble  in  any  other  part  of  the 
bod}^,  particularly  if  their  disease  is  accompanied 
by  a  discharge.  But  in  this,  as  in  all  preventive 
measures,  if  the  habit  of  carefulness  be  formed 
during  times  when  there  is  no  emergency,  in  the 
time  of  emergency  its  effectiveness  will  be  exer- 
cised automatically. 

Posture. — The  influence  of  posture  upon  the 
liealth  and  development  of  the  eye  is  a  most  im- 
portant consideration.  Nothing  is  more  detri- 
mental to  the  eye  and  more  destructive  of  good 
vision  than  the  pernicious  habit  of  poring  over 
one's  work  with  one's  head  bent  down  almost 
touching  the  knees  Avhich  is  so  common,  especially 
among  children^  in  whom  it  is  particularly  injuri- 
ous. Such  a  posture  induces  general  congestion 
of  the  eyeball,  with  an  overproduction  of  its  se- 
cretions, stretching  the  choroid,  and  producing 
choroiditis  and  progressive  myopia  by  tlie  elonga- 
tion of  the  eyeball.     ]N"ot  infrequently  a  progres- 


HYGIENE    OF   THE   EYE  223 

sive  process  has  been  started  in  this  way  tliat  lias 
eventually  destroyed  the  eyesiglit. 

Myopes  are  particiUarly  ^ilty  of  this  habit, 
and  of  course  in  tliose  whose  eyes  are  already 
myopic  the  mischievous  effects  of  the  habit  are 
most  pronounced. 

Every  person,  and  especially  myopes,  therefore, 
should  in  reading  or  sewing  or  any  other  type  of 
near-work  form  the  habit  of  sitting  up  straight 
and  holding  the  work  at  a  comfortable  distance 
fi'om  the  eyes.  If  their  refraction  will  not  permit 
of  their  doing  this,  then  there  is  something  wrong 
with  their  refraction  and  they  should  obtain  such 
correction  as  will  enable  tliem  to  carry  out  the 
practice  recommended. 

Lighting.    ^ 

All  of  the  plans  and  devices  that  ever  have  been 
or  ever  can  be  propounded  for  the  best  possible 
lighting  of  rooms  resolve  themselves  into  applica- 
tions of  the  basic  principle  that  the  light  is  for  the 
purpose,  not  of  calling  attention  to  itself,  but  of 
revealing  objects.  The.  whole  problem  of  light- 
ing consists  in  attaining  a  light  which  shall  ob- 
trude itself  upon  the  eye  to  the  least  possible  ex- 
tent, and  at  the  same  time  disclose  the  objects  to 
be  viewed  with  the  greatest  possible  clearness. 

The  applicai  icii  of  tliis  ]n'inei))le  necessarily 
implies  the  reflection  of  tlie  light  from  tbe  object 
to  the  observers  eye.  Its  working  out,  therefore. 
involves  tbe  ennsidoration  of  two  j^oucimI  ff^iitures: 


224  REFRACTION 

(1)  the  source  and  nature  of  the  light,  and  (2) 
the  course  traversed  by  the  rays  from  their  origin 
to  the  object  and  thence  to  the  observer.  All 
questions  concerning  the  light  come  either  under 
the  head  uf  its  illuminating  qualities  or  that  of 
its  propogation. 

ILI.UMI.^^ATIO^^ — la  fulfillment  of  the  require- 
ment already  enunciated^  that  the  light  shall  at- 
tract as  little  attention  as  possible  to  itself,  it  is 
essential  that  it  be  as  diffuse  as  practicable.  This 
is  the  quality  which  the  layman  usually  means 
when  he  calls  a  light  '^soit,"  and  it  has  its  realiza- 
tion in  the  natural  light  of  a  clear  day  out  of  the 
immediate  path  of  the  sun's  rays.  Gould  points 
out  that  the  ideal  method  of  attaining  this  con- 
dition with  artificial  light  would  be  to  have  the 
source  or  sources  of  illumination  concealed  from 
view  and  arranged  so  as  to  exercise  a  uniform 
diffusion  of  the  light  proceeding  from  them 
throughout  the  space  designed  to  be  lighted.  This 
effect  is  nowadays  attained  by  the  so  called  "in- 
direct" system  of  lighting,  in  which  the  light  is 
reflected  from  ceilino-  and  walls :  and  this  is  the 
ideal  system. 

Daylight,  then  (but  not  direct  sunlight),  is  the 
ideal  illumination,  in  spite  of  all  foolish  asser- 
tions to  the  contrary,  and  it  should  be  admitted  to 
the  room  in  such  a  way  as  to  render  its  distribu- 
tion as  diffuse  and  uniform  as  possible. 

Daylight  being  inaccessible,  or  for  any  reason 
undesirable,  the  next  best  substitute  is  the  indirect 


HYGIENE    OL'   THE   EYE  225 

liditins"'  just  referred  to.  After  tliat  the  next  best 
is  a  wliiie  or  sliglitly  yellow  artilieinl  light,  .sur- 
rounded hy  a  ^^pherieal  globe  of  opacjuc  glass  for 
the  purpose  of  dilTusion.  The  globe  need  not  he  a 
complete  sphere,  for  it  is  rarely  necessary  to  direct 
any  of  the  light  upwai-d.  hut  it  sliould  be  of  a 
spherical  curvature,  so  as  to  obtain  a  uniform  dif- 
fusion. Uniformity  of  distribution  is  best  attained 
by  a  correspondingly  uniform  distribution  of  the 
sources  of  light  throughout  the  room,  in  prefer- 
ence to  grouping  them  in  the  center.  This  must 
be  done  with  due  regard  to  the  matter  of  inten- 
sity, of  which  we  shall  treat  directly,  and  of  the 
purposes  for  which  the  room  is  to  be  used. 

Intensity. — The  intensity,  or  what  the  layman 
calls  the  "strength,''  of  the  light  is  a  matter  whose 
adjustment  depends  largely  upon  the  plirpose  for 
which  it  is  desired,  and  one  in  which  the  most 
paradoxical  contradictions  are  perpetrated  in  act- 
ual practice.  One  does  not  require  as  much  in- 
tensity of  light  for  a  dancing  party  or  a  reception 
as  one  does  for  reading  or  sewing,  hut  as  a  rule 
the  dancing  or  reception  room  is  lighted  far  more 
brilliantly  :liau  the  reading  or  sewing  room;  in- 
deed, the  former  is  usually  lighted  far  too  hril- 
lianlly  for  the  peace  and  welfare  of  the  eyes,  wliil(> 
ihd  latter  is  generally  just  the  reverse. 

Too  great  intensity  of  light  is  injurious  to  the 
eyes  chiefly  by  reason  of  its  exhausting  effects 
upon  the  retina,  and  also  because  it  reveals  more 
detail    than    is   necessarv    in    the    objects    viewed 


226  ftEFRACTION 

and  thus  induces  other  forms  of  eye-strain.  Too 
little  intensity,  on  the  other  hand,  is  harmful  be- 
causd  of  the  necessity  which  it  imposes  on  the 
eye  of  close  aiDplication  and  consequent  muscular 
strain. 

For  ordinary  purposes  of  vision,  such  as  dan- 
cing, dining,  receiving,  etc.,  the  intensity  should 
be  just  sufficient  to  afford  a  general  view  of  ob- 
jects and  people  such  as  one  desires  on  such  occa- 
sions, and  subdued  enough  to  be  restful  to  the  eye 
— i.  e.,  not  to  reveal  unnecessary  and  nagging  de- 
tail. For  purposes  of  reading  and  sewing  and  sim- 
ilarly close  work,  the  intensity  should,  of  course, 
be  greater — sufficient  to  reveal  clearly  all  the  nec- 
essary details  of  the  work  without  undue  applica- 
tion, but  not  enough  to  obscure  these  details  b}'' 
excessive  reflection  or  to  tire  the  retina  by  over- 
stimulation. 

It  is  estimated  that  for  the  former  require- 
ments an  intensity  of  32  candle  power  per  1,000 
cub.  ft.  of  space  is  about  right,  while  for  close 
work  it  should  reach  G4  candle  power. 

Modes  of  Light. — A^iewcd  from  the  standpoint 
of  the  foregoing  qualifications,  it  will  at  once  be 
seen  that  daylight  is  easily  the  most  desirable 
mode  of  light.  In  addition  to  its  other  superiori- 
ties it  possesses  the  distinctive  advantage  of  fur- 
nishing a  white  light,  which  is  not  perfectly  at- 
tainalile  by  any  other  method  of  lighting,  and 
which  is  ilie  most  conducive  to  acuity  of  vision. 
Its   chief   disadvantages   are   its  unreliability,   its 


HYGIENE   OP  THE  EYE  227 

inadequate  duration,  and  the  difficulty  under  mod- 
ern crowded  conditions  of  admitting  it  in  sufli- 
cient  quantities.  These  circumstances  make  it  ab- 
solutely impossible  to  depend  upon  it  for  illumi- 
nating purposes  in  these  days  of  competitive  effort. 

Artificial  modes  of  light  possess  degrees  of  de- 
sirability corresponding  to  the  proximity  with 
wliich  they  realize  tlie  advantages  of  daylight,  and 
the  extent:  to  wliich  they  avoid  the  disadvantages 
peculiar  to  artificial  lighting.  Chief  among  these 
disadvantages,  common  to  all  artificial  methods, 
are  tlie  unsteady  character  of  their  light  and  the 
elfects  produced  by  combustion  upon  the  atmos- 
phere. 

Electric  Light. — The  mode  of  'artificial  light 
which  comes  nearest  to  fulfilling  tliese  conditions 
is  undoubtedly  the  electric  incandescent  light.  It 
possesses  the  following  distinctive  and  positive  ad- 
vantages; it  gives  an  almost  white  light;  it  burns 
with  comparative  steadiness;  it  yields  tlie  greatest 
degree  of  intensity  for  the  least  bulk;  its  intensity 
is  most  easily  measured  and  regulated.  It  lias, 
moreover,  the  following  negative  virtues:  it  does 
not  vitiate  the  air;  it  does  not  affect  temperature 
or  humidity  to  any  appreciable  extent;  it  carries 
with  it  no  danger  of  fire. 

Incidentally  it  may  be  added  that  it  is  the  most 
economical  light,  for  the  reasons  above  set  forth, 
both  in  tlie  matter  of  diroct  production  and  also  of 
physical  liealth. 

Qas. — Next  to  the  eh-ctric   incandescent  lamp. 


228  BEFBACTION 

illuminating  gas  with  some  form  of  mantle  burner 
furnishes  the  most  desirable  artificial  light.  By 
means  of  these  burners  iruu-li  of  the  objection  to 
the  color  of  tlie  liaiuc  and  its  unsteadiness  is  over- 
come, ami  its  intensity  is  enlumced.  However,  it 
still  retains  the  disadvantages  of  vitiating  the  at- 
juosphere,  affecting  temperature  and  humidity,  and 
affording  constant  danger  of  fire  and  explosion,  to 
which  may  be  added  the  ])ractical  impossibility 
of  ever  obtaining  a  good  grade  of  gas.  Acetylene 
gas  is  an  impr(/vement  in  every  way  u])on  ordinary 
coal  gas. 

Lamps. — In  the  absence  of  facilities  for  either 
electric  or  gas  lighting,  one  must  of  course  use  oil 
lamps.  They  are  very  undesirable  things  at  best, 
and  all  one  can  do  is  to  lessen  their  objection- 
able features  as  much  as  possible  by  employing  a 
high  grade  of  kerosene — i.  e.,  one  with  a  high 
flasli  point,  a  well-made  air-tight  lamp,  a  good 
circular  wick,  and  a  first-class  chimney  surrounded 
by  an  opaque  white  globe.  An  ordinary-sized  room 
will  require  at  least  two  such  lamps  of  large  size 
to  adequately  light  it  for  reading  or  sewing. 

All  the  modes  of  artificial  lighting  above  dis- 
cussed are,  of  course,  subject  to  the  requirements 
of  diffusion  and  intensity  previously  set  forth. 

Reflection. 

If  the  conditions  ol'  ditfusion.  disti'ibution  and 
intensity  above  set  forth  could  in  all  cases  be 
id('al1\-   realized,   then   there  would   l»e  no  need  to 


HYGIENE    OF   THE   EYE  229 

discuss  the  aspect  of  the  subject  wliicli  coucerns 
rellectiou.  For  with  ditt'used  light,  whetlier  it  be 
daylight  or  artificial  light,  uniformly  distributed 
throughout  the  desired  space,  and  of  a  net  inten- 
sity proper  to  the  required  purpose,  the  light  would 
be  uniformly  reflected,  as  to  direction  and  inten- 
sity, from  any  point  included  in  the  illuminated 
room.  Unfortunately,  however,  such  ideal  condi- 
tions are  rarely,  if  ever,  attained,  and  it  is  almost 
always  necessary  to  make  specific  provision  for 
the  proper  incidence  and  reflection  of  light,  in 
the  required  degree  of  intensity,  from  some  se- 
lected portion  and  aspect  of  the  building  or  space. 
This  is  accomplished  principally  by  adjustment 
of  the  postural  relations  of  the  sources  of  light 
to  those  of  the  objects  and  observers,  either  by 
arrangement  of  the  windows  or  artificial  liglits  or 
by  arrangement  of  the  objects. 

Position'  of  Light. — This  is  determined  ac- 
cording 1o  the  principles  of  diffusion,  intensity, 
and  reflection  already  enunciated,  and  with  regard 
to  tlu"  illuminating  purpose  of  the  light. 

It  is  evident  on  nil  of  these  grounds  that  tlie 
object  to  be  viewed — the  book  to  be  read,  for  ex- 
ample— should  not  be  held  l)etween  the  person 
and  the  source  of  light,  or  that  tlie  source  of 
light  should  he  in  front  of  the  i)erson  at  all.  For 
a  light  in  fi'Diit  of  the  o])server  calls  attenti(m  to 
itself,  exhausts  the  retina,  is  not  reflected  from 
the  object  and  therefore  does  not  illumine  it,  and 
in  the  case  of  sewing  or  writing  casts  a  shadow 


230  BEFRACTION 

toward  the  worker — in  all  of  these  respects  vio- 
latiii*^  tlie  canons  of  good  illumination. 

It  is  ec[ually  evident  that  the  source  of  light 
ijnmediately  behind  the  observer  will  not  fulfil 
the  required  conditions,  for  in  that  case  the  ob- 
server's own  body  will  be  interposed  between  the 
source  of  light  and  the  object,  so  that  the  light 
cannot  reach  the  object  to  be  reflected  from  it,  and 
all  of  the  room  will  ])e  illuminated  except  the  very 
portion  -w'here  illumination  is  desired — namely, 
where  the  object  is — thus  creating  a  very  annoy- 
ing and  tiresome  contrast. 

The  source  of  light  directly  to  the  side  of  the 
observer  is  as  bad  as,  if  not  worse  than,  directly 
in  front.  In  this  position  the  rays  of  light  strike 
the  object  at  an  extremely  oblique  angle  and  are 
reflected  at  an  equally  oblique  angle  to  the  other 
side  of  the  observer;  indeed  many  of  them'  pass 
laterally  between  himself  and  the  object.  Only 
those  reach  him  from  the  object  which  are  trans- 
mitted to  it  indirectly,  like  the  cushioning  of  a 
billiard  ball.  In  addition  to  these  faults  of  re- 
flection, the  light  from  the  source  of  illumination 
is  striking  his  eye  directly  at  a  lateral  inclination, 
and  irritating  his  retina  in  a  peculiarly  aggra- 
vating wa}'. 

The  Correct  Position. — The  most  satisfactory 
position  for  the  source  of  light  is  midway  between 
the  rear  and  the  left  lateral ;  sufficiently  in  the 
roar  to  nvoid  any  direct  rays  falling  on  the  eye; 
sufficiently  to  the  side  to  clear  the  observer's  own 


JIYGIENE    OF    THE    EYE  2:51 

bod}-;  aiKl  suificieiitk  high  to  clear  his  arm  and 
shoukler.  In  this  position  the  light  falls  upon 
the  object  at  a  slightly  oblique  angle,  but  not 
siillicicntly  oblique  to  prevent  it  from  reaching  the 
observer's  eye,  especially  if  the  object  be  appro- 
})riately  tilted  toward  the  source  of  light. 

The  retina  is  now  receiving  no  direct  light,  none 
in  fact  but  what  is  properly  reflected  from  the 
object,  and  no  shadow  is  cast  between  the  observer 
and  the  object.  When  the  source  of  light  is  in  a 
similar  position  over  the  right  shoulder  a  shadow 
is  thrown  between  the  observer  and  the  object  un- 
less he  moves  the  object  to  the  left,  in  doing  which 
he  takes  it  out  of  the  fixation  field  of  the  right,  or 
dominant,  eye.  Some  persons  are  sinistrocular, 
or  ^'left-eyed,''  and  for  these  persons  the  proper 
position  for  the  source  of  light  is  over  the  right 
shoulder. 

Influence  of  Position  on  Intensity. — In  ar- 
rangements of  position  such  as  we  are  contom- 
j dating;  the  source  of  light  must  not  he  too  far 
from  the  oljject  to  illumine  it  in  sufficient  detail, 
noi-  must  it  he  so  near  as  to  unnecessarily  exag- 
gei-ate  detail  and  tire  the  retina.  It  should  be  suf- 
ficiently far  removed  to  comfortahly  reveal  just 
the  amount  of  detail  required  for  the  purpose  in 
hand. 

Influence  of  Position  on'  DrFFUSiON. — The 
pame  remarks  apply  to  the  diffusion  of  light.  If 
the  source  of  light  he  too  near  the  ohject  the  light 
will  not  be  properly  diffused  by  the  time  it  reaches 


2L'2  REFRACTIOX 

the  object;  if,  on  the  other  hand^  it  be  too  far 
removed,  diffusion  will  be  so  great  as  to  weaken 
the  intensity  and  the  reflective  power. 

If  the  source  of  light  be  a  window  (daylight) 
and  it  be  out  of  the  direct  rays  of  the  sun,  on ; 
can  hardly  be  too  near  for  either  intensity  or 
diffusion. 


GLOSSARY. 

Accoinniodation. — The  power  of  changing  the  focus 
of  the  ej'e.  by  contracting  tlie  ciliary  muscle. 

Ametropia. — An  abnormal  condition  of  refraction. 

Amplitude. — The  amount  of  nervous  energy  neces- 
sary to  perform  a  function. 

Angle  Alpha. — The  angle  formed  at  the  nodal  point 
by  tlie  optic   and   visual   axes. 

Ang-le  Gamma. — The  angle  made  by  the  visual  axis 
and  a  line  drawn  from  the  object  through  the  center 
of  rotation. 

Anisometropia. — A  difference  of  refraction  in  the 
two  eyes. 

A.stlienopin. — Weakness   of  the   visual   apparatus. 
Astigmati.sm. — Inability  to  focus  the  rays  at  a  point. 
Binoetilar. — Relating    to    both    eyes    used    simultane- 
ously. 

Centering-. — Placing  a  lens  before  the  eye  so  that 
the    visual    axis    passes    through    the    principal    axis    of 

the    lens. 

Cliamber.s. — Divisions    of    the    interior    of   the    eyeball 

containing   the    liumors. 

Clioroid. — The  vascular  tunic  of  the   eye-ball. 

Chromatic  Test. — A  refractive  test  made  with  a  lens 
which   separates  up   the   ray  into  the  primary  colors. 

Ciliary. — The  circular  muscle  of  the  eye  which  sur- 
rounds   the   pupil. 

Concave. — Diverging   refracted   rays. 

Convergence. — The  directing  of  the  visual  axes  of 
the  two  eyes  toward  a  point  nearer  than   infinity. 

Convex. — Converging  refracted  raj'S. 

Cornea. — The  smaller  spherical  segment  of  the  eye- 
ball. 

Cyeloplegric.^An  agent  for  paralyzing  the  ciliary 
muscle. 


GLOSSARY  233 

Cylinder. — A  lens   which   is   a  segment   of  a  cj'linder. 

Diopter. — The  unit  of  refracting  power,  namely,  that 
of  focusing  parallel   rays  at  a  distance  of  one  meter. 

Dise  (optic). — The  raised  circular  spot  where  the 
optic  nerve   enters   the  retina. 

Einiiielropin. — A    condition   of   normal   refraction. 

Kye.straiu. — The  nervous  effects  of  abnormal  re- 
fraction. 

Far  Point. — The  furthest  point  at  which  a  com- 
pletely relaxed  eye  can  clearly  discern  an  object 
whose  size  corresponds  to  the  visual  angle. 

Finite  Kays. — Rays  which  originate  within  six  me- 
ters   of    the    observer. 

Focal  Distance. — The  distance  between  the  refract- 
ing  surface   and    the   focus. 

Focal  I^engtli. — Focal  distance  as  applied  to  a  lens. 

Focal  I*oint. — The  point  at  which  refracted  rays  are 
brought  to  a  focus. 

Fovea  Centrali.s. — The  most  sensitive  point  in  the 
retina,   situated   in   the  center  of  the  yellow  spot. 

Fundus. — Tlie  retina  as  seen  through  the  ophthal- 
moscope. 

Heteroplioria. — A  condition  of  unequal  power  in  the 
ocular   muscles. 

Humors. — The  licjuid  and  semi-liquid  contents  of  the 
eyeball. 

Hypernietropia. — A  condition  of  refraction  in  which 
the  rays  are  carried  behind  the  retina  to  focus. 

Imbalance. — See  Heteroplioria. 

Index  of  Kefraction. — The  refracting  power  of  a 
medium  as  compared  with  that  of  air,  atmospheric 
refraction   being  ligured  as   1. 

Infinite  Rays. — Rays  which  originate  more  tiian  six 
meters    from    the    ol)sorver. 

liatent  Hypermetropia. — Hypernietropia  due  to  an- 
atomic ciianges  in  the  ciliary. 

Macula    laitea. — See   Yellow   Spot. 

Maddox  Rod. — A  device  for  testing  tlie  muscles  of 
the    eye. 

Manifest  Hypermc<ro|>Ia. — Hypermetropia  due  pure- 
ly to  refiactive  conditions  and  easily  demonstrable  by 
ordinary   tests. 

Meridian. — A    line   of   curvature   of   the  cornea. 

Meridians,  Chief. — Tiie  meridians  of  greatest  and 
least  convexit\-   icspectively  of  the  eye. 

Myopia. — A  condition  of  refraction  in  which  paiallel 
rays  are  focused   in   front  of  the  retina. 

Xear  Point. — The  nearest  point  at  which  the  com- 
pletely accommodated  eye  can  clearly  perceive  an  ob- 
ject whose  size  corresponds  to  the  visual  angle. 


234  REFRACTION 


INoKsitive  C'onvtTseufe. — Tlie  power  of  the  external 
recti   muscles  to  pull  the  eyeballs  outward. 

Xodsil  I*oiiit. — The  center  of  the  refracting  system  of 
the  eye,  through  which  all  the  rays  pass  as  they  cross. 

Oplitiiiiliiio.oicope. — A  mirror  for  examining  the  fun- 
dus   of    the    eye. 

Opiitlinlinoscopy. — The  use   of   the   ophthalmoscope. 

Optif  Axis. — An  imaginary  line  drawn  through  the 
nodal   point  to  the  inner  side  of  the  yellow  spot. 

I'ri.siii. — A   pyramidal    shaped   lens. 

I'reshyopia. — Hypermetropia  due  to  old  age. 

I'riiu-ipal  Axix. — The  imaginary  line  along  which  a 
ray  travels  which  enters  a  medium  perpendicularly  to 
its    surface. 

I'o.sitive  Coiiverg-ence. — The  power  of  the  internal 
recti   muscles   to   pull   the   eyeballs   inward. 

Rang-e. — The  maximum  distance  which  a  function 
can   be   exerted. 

Retincseopy. — The  shadow  test  by  means  of  a  retino- 
scope. 

Strabl.simi.s. — Squint. 

A'i.sual  Angle. — The  angle  formed  by  two  lines  drawn 
from  the  extreme  boundaries  of  the  object  looked  at 
through   the   nodal   point. 

V'iNual  Axis. — An  imaginary  line  drawn  from  the 
yellow   spot,    through   the   nodal   point,   to   the   object. 

Yello^v  Spot. — The  most  sensitive  spot  on  the  retina, 
situated  in  the  centre  of  the  retina,  to  the  outer  side 
of  the  disc. 

METRIC  EQUIVALENTS. 

1    meter   =   3937    inches.      Approximately    40    inches. 
1    antimetre  =  .40   inch.      Approximately    i/^    inch. 

1  millimetre   =   .04   inch.      Approximately    1/25    inch. 

COMMOM.Y    ISED     DISTANCES. 

6   meters  =  approximately  20  feet, 

n   meters  =  api>roximately  16  feet. 

4    metnrs  =  ;i  pin'oximately  11  feet. 

3    meters  =  api)i  oximalely  ]0  feet. 

2  meters  =  approximately  7  feet. 

1    meter  =  approximately  40  inches. 


INDEX 


Alicnal  inn,    Spluiiial 31 

Ahiiuriual    Acruiiiiiiutlalioii.  .  .174 

— Com ( if^TiRe     175 

Absolute'  and    JJiiiocular G7 

— Negative  Convergence.  ...  74 
AceoHiinodation     Abnonnal.  .174 

— Anii)litude  of 00 

— and    Convergence 63 

— Observer's    lOG 

— Range  of 05 

Achromatic     Bi-Focals 200 

Acuity'  of  Vision 42 

Acute    Conjunctivitis     ISO 

Alpha     Angle 42 

Ametropia   4'J 

— Ophthalmoscope   in 103 

Anietropic     Eye 49 

Amplitude  of  Accommodation  00 
— of  Negative  (.'onvergence.  .  74 
— of  Positive  CVmvergence.  .  .  74 
Anatomic  Construction   of...    35 

Angle   Alpha 42,   154 

— Gamma    42 

— of    Incidence     15 

— of    liefiection    15 

— Visual      42 

Anisometropia     50 

Anterior    Focus,     Principal..    32 

Apparent     Strabismus     153 

Asthenopia      109 

— Accommodative     171 

— Symjitoms    of     109 

— Varieties     of     1 70 

Astigmatic  Hand    138 

Astigmatism      50 

— C'omi)ound      132 

— C'orrection      127 

—Mixed    134 

— Ophthalmosropy     in     130 

— OjihthalMiosctipe    in     105 

— Tfetinoscopy     95 

— Retinoscopv     in     130 

— Simple     131 

— Stenopaic   Slit    in     129 

— Test    Tvpe    in     1  2S 

— Wheel  Ti'st   in    12S 

Atrophy.    Optic    IRS 

Axes  and   Points    3S 

— of     the     K.\-e     41 

Axis   Optic    41 

— Princiiial     29,   41 

—Visual     41 

Bi-Focals      190,   204 

• — Achromatic     206 


— Cemented     2(t5 

- — (jlround     2iM 

Binocular  and   Absolute    ....    07 

JJridges    201 

Jiridge   Measurements    202 

Causes    of    Imbalance    159 

Cemented    Bi-Focals    205 

C'enter,    Optical    24 

Chambers     37 

Chief     Meridians     53,   127 

Children's   E\es    217 

Choked  Disc" 187 

Choroiditis      184 

Chronic  Conjunctivitis    181 

Chromatic   Test    113 

Ciliary     37 

Circles  of  Diffusion   33 

Close   Application    215 

Cold    in    the    Eye    221 

Colors,    Glittering    219 

Commonly  Used  Distances.  .234 
Compound  Astigmatism  ....132 
Computing  the  Correction..  94 
Concave    Reflections   of    Light  24 

Concentration    of    Light 92 

Conjunctivitis     180 

— Chronic 181 

—Purulent    181 

Convergence    03 

• — Abnonnal     175 

— Absolute  Negative   74 

— Amplitude  of  Negative.  .  .  74 
— Amplitude    of     Positive.  .  .    74 

-—Insufficiency    of    75 

— Measure  of    72 

— Range    of     09 

— Ratio    of     74 

Convex   Ix^ns  for   Relaxation  .1  45 

Cornea.     The 3(i 

Correction,    Comi>uting   the.  .    94 

— of    .\stigmatism 127 

— of    ITvpermetropia     113 

— of    Myopia    121 

Correct    Position    of    Light.. 230 
Decenfered    Ij<>nses,    Prescrib- 
ing      209 

Decentoring    of    Lrns(s     ....200 

—of  Reading  GIass<'S 212 

■ — -Equivalents     211 

-   Afethods    of     210 

-Pnsm.Tfic   Effect    of    207 

Detached     Retina     187 

Diagnosis  of  Strabismus  ...15.'5 
Dioptric   System    48 


236 


INDEX 


Dioptrisni    48 

Dittusion,    Circles    ot     S'd 

Disc     38 

— View    of    110 

Diseases  of  the  Eye  Connect- 
ed    with     Disturbances     ot 

\'ision    179 

Distance   of   Lens   from    Eye. 212 

— Principal     30 

— Principal  Posterior    32 

— Tj'pe     144 

Distances    Connnonlv    rsed..234 

Distant    Tvpe    Test." 114 

Dust    in    Eyeball 220 

Edges,  Oblique 95 

■ — of   Shadow    94 

Electric    Light     227 

Emnietropia     49,   88 

Emmetropic  Point  of  Revers- 
al        84 

Errors,     Estimation     of     Re- 
fractive       100 

— of  Refraction    .50 

— Possible     102 

Examination      79 

— of  Refractive  Errors   160 

Excessive  Illumination 218 

External    Rectus    45 

Eye,    The    35 

— Anatomic    Construction    of  35 

— at  Rest    63 

— ^Axes   of    41 

—Cold    in     221 

— Diseases    of     170 

— Distance   of   Lens   from    ..212 

— Hygiene     of      215 

— Infection   in    221 

— Glasses     203 

— Muscles    of     44 

— Refraction  of    47 

— Size   of    204 

— Strain,    Reflex    Effects   of.. 172 

Eveball     3.),   220 

— Dust    in     220 

Eves,     Children's     217 

Far    Point     04,   73 

Fitting    Glasses    191 

F'ocal    Ijcngth     25 

— Length  of  Mirror 93 

— Point,   Negative    31 

— Point,    Positive    31 

— Point,    Principal    30 

Focus,    Principal    25 

— Visual    20 

Formation   of    Images    32 

Fundus.    Observation   of    ....102 

Gamma    Angle    42 

Gas     Light     227 

Glasses,  Decentering  of  Read- 
ing      212 

—Fitting    191 


— Periscopic      196 

— Prescribing     198 

Glaucoma     184 

Glittering  Material  or  Colers.219 

Glossary     232 

Gradual  Relaxation   116 

Ground    Bi-Focals     204 

Half   Lenses    206 

Heterophoria,    Varieties    of..  158 

Humors      37 

Hvgiene    of    the    Eve    215 

—Retinal    ' 218 

II\-]iermetropia      ....49,   51,      88 

• — Correction    of .  .113 

— Ophthalmoscope    103 

Hypermetropic    Point    of    Re- 
versal         85 

Hvpermetropia,      Retinoscopy 

in     117 

Hypei-opia,    Latent     146 

— Measure    of     116 

H^peropic    Eye    49 

Illumination      224 

— Excessive     218 

—Poor 218 

Images,    Formation    of    32 

Image    Reflected     22 

—Visual    26 

Imbalance     153 

■ — Causes   of    159 

— Muscular      158 

—Tests    for    163 

— Treatment  of    166 

Incidence,    Angle    of     15 

Index   of   Refraction    48 

Indirect    Ophthalmoscopy.  .  .  .106 

Infection    in    Eye    221 

Inferior    Oblique    45 

— Rectus     44 

Influence  of   Position   on   Dif- 
fusion  of  Light    231 

— of  Position  on  Intensitv  of 

Light      ;...231 

IiisuHiciency   of   Convergence.    75 

Intensity    of    Light ...  223 

Internal  Rectus   44 

Intei-pretation    of    Shadow.  .  .    88 

Interstitial    Keratitis     183 

Intrinsic  Muscles    44 

Iris     30 

Iritis    183 

Keratitis     Interstitial 183 

Lamps     228 

Latent    Hyiieropia    146 

Laws   of    Reflection 16 

Lens     36 

— Focal   Ijcngth   of    58 

— Focal   Point   of    57 

— Measurement   of   Accommo- 
dation        66 

Lenses     57 

• — Bi-Concave     60 


INDEX 


237 


— Bi-C'onvpx      00 

— (.'oncav(>-(  'onvpx    60 

— Convi'if^iiif;'    Coiifavo     Con- 
vex         60 

—Cylindrical     16,     59 

— Decenteiing  of    206 

— I)iiiiinislunf4'  Ett'oct  of  Con- 
cave       150 

- — Diverj^inj;'   Concavo-Cnnvcx.    60 

— Crindinj;-    60 

— foi"    Myopia     122 

— Piano-Concave     60 

— Piano-Convex    60 

— Prescrihiiif^'  Decentered    ...209 

—Spherical      58 

Lifiht      9 

— Al)sorption  of    10 

—Color   of    13 

— Concentration   of    92 

—Concave  ReHection  of 24 

— Correct    I'osition    of    230 

• — Divergence  of  Kays  of .  .  .  .    14 

— Dynamics    of    11 

—Effects  of  on  Matter 12 

—Electric     227 

- — Finite   Ray.s   of    15 

— Force  of 12 

— for  Retinoscope    80 

—Gas    227 

— Generation    of     9 

— Geometries   of    14 

— -Infinite  Hays  of    14 

— InHucnce     of     I'osition     on 

Diffiisi(m    of     231 

— Influence     of     Position     on 

Intensity  of    231 

— Intensity   of    13,    223 

— Linear   Propagation   of....    14 

—Modes    of    226 

— Nature    of    9 

— Oscillatory  Velocity  of....    12 

—Position   of    92.    229 

— Kavs     14 

—Reflection    of    10,    22 

—Refraction  of    28 

— Sources     of      9 

— Transmissir)n   of    9 

— Velocity   of    U 

— \'il)rations     13 

Lighting    223 

•Maddox  I'upil   Localizer 192 

— i{..d    I(i3 

— Use  of    165 

Malignant      I'rogressive      My- 
opia       148 

Measure    of    Convergence    ...    72 

— of     nyp(roi)ia     116 

Meridians,  Testing 136 

Mitiiod    of    Estimation    !)5 

Nfrtiiods    of     Dccentering.  .  .  .21(1 

.Met lie    .\ngle    73 

— E.piivalents     234 


Minus    Lenses     53 

Mirror.     Focal    Length    of .  .  .    93 

— Movements  of 93 

Mixed    Astigmatism     134 

Modes  of   Light    22(1 

Movements  of   Mirror    93 

Muscles  of  the  E\e 44 

Muscular    Lnhalancc    158 

— Mechanism     45 

Myopia .4!),   52,  8!t 

— Correction    of     121 

— Distant    Type    Test    for... 122 

-Lenses  for 122 

— .Malignant    Progressive    ...148 

— (Ophthalmoscope     in     104 

— Retinoscopy    in     123 

Myopic    Point    of    Reversal..   86 

Near    Point    65,   74 

Negative  Focal  Point 31 

Neuritis,    Optic    187 

Nodal     Point      39 

Normal    .\mplitude   of    .\ccm- 

modation     67 

Observe'r's  .\ccommodation  .  .  .106 

— Refraction     105 

Oblique   Edges    95 

Observation  of  Fundus    102 

Opacity   10 

Operator,   Position  of    KM 

Ophthalmoscope     97 

— in   .\metropia    103 

■ — in   Astigmatism    105 

• — in    Hypermetropia     ......103 

— in     Myopia      104 

Ophthalmoscopy     97 

—Direct     l<l(i 

— in    .\stigmatism     139 

—Indirect    106,   110 

— Indirect  in  .\stigmatism  .  .1 11 
— Indirect  in  Emmetropia  .  .  llo 
— Indirect  in  Hypermetropia  .  11 1 

— Indirect  in  Myopia    Ill 

Optical    C-enter    24 

Optic  Atrophy    188 

— Neuritis     187 

Optics    19 

Points    38 

Panmis    182 

Paralytic    .Strabismus    157 

Periscopic  (Jlasses    196 

Pin    IToK'  T.St    143 

Plus    Lenses    52 

Point  of   l{eversal    83 

Poor    Illuminations    218 

Position    of    Light     !i2.   229 

Positive    Focal     Point     31 

Pos.sible     Errors     li»2 

Posterior    Focus,    Principal..    .30 

I'osture,   IjiHuence  of    222 

Presbyopia     54 

P  r  e  s  c  !•  i  b  i  n  ir    I  )e(  entered 

Lenses      209 


238 


INDEX 


— of    Glasses    198 

Principal    Anterior    Focus...   32 

— Axis    29,  41 

— Distance    30 

■ — Focus    25,   41 

— Focal   Point    30 

—Point   3!J 

— Posterior  Focus   31 

Prism  Tests 70,   1G6 

Prismatic  Test    127,  143 

— Effect  of  Decentering   ....  207 

Projection      23 

Pterj^gium     220 

Punctum    Remotum     64 

Pupil   36 

— Localizer 192 

Pupillary    Distance    192 

Purulent    Conjunctivitis 181 

Jiange   of   Accommodation.  .  .    65 

— of    Convergence    69 

Ratio     of     Convergence     and 

Accommodation    75 

Rays    14 

— Divergence  of : 14 

— ^Finite    15 

—Infinite    14 

— of  Direction    33 

Real    Strabismus    154 

Reading     Glasses,      Decenter- 
ing of    212 

Reason      of      Movements      of 

Shadow    82 

Reflected    Image    22 

Reflection     15,  228 

— Angle  of   15 

— Laws   of    16 

— of  Light    22 

Reflex  Effects  of  Eye  Strain.  172 

Refraction     16 

— -Degrees  of    29,  50 

— Index   of    17,   48 

— Observer's     lOS 

— of   Light    28 

— '.^f  the  Eye    47 

— Spherical     29 

Refractive  Imbalance    159 

Rela.xation,  Convex  Lens  for.  145 

—Gradual     116 

Rests,   Frequent    216 

Retina     37 

— Detached      187 

Retinal  llvgiene 219 

Retinitis      186 

JJetinoscojio     78 

— in     Astigmatism     1)5 


■ — Principal    of     78 

Retinoscopy    77 

— in  Astigmatism    136 

— in   Hypermetrupia    117 

— in    Myopia    123 

Reversal,     Emmetropic    Point 

of     84 

- — ^Hypermetropic    Point    of..    85 

— Myopic   Point  of    86 

Shadow,   Inteii)retation   of .  .  .    88 

— Retinoscopy     82 

— Test 77 

Simple  Acute  (A)njunctivitis.  180 

— Astigmatism      131 

Six  Meter  Method    93 

Size   of   Eye    204 

Skiascop3-    77 

Spectacles      145 

Spherical    Aberration    31 

— Refraction     29 

Spot,    Yellow    38 

Stenopaic     Slit    in     Astigma- 
tism      129 

Strabismus    and    Imbalance.  .153 

— Apparent      153 

— Diagnosis  of    155 

— Paralytic     157 

— Real     154 

— Treatment   of    157 

Studs      204 

Superior  Oblique   45 

— Rectus      44 

Temples 201 

Test,    Maddox     163 

—Pin     Hole     143 

— Prismatic     143 

— Type  in  Astigmatism 128 

—Types     43 

— Chromatic    113,   121 

— Distant  Type    114 

Tests  for  Imbalance    163 

— Separate     141 

Trachoma    182 

Transparency    10 

Treatment   of   Imbalance    ...166 

— of  Strabismus 157 

Varying    Velocity     96 

View  of  the  Disc    110 

Visual   Focus    26 

Visual    Image    26 

Visibility     19 

Vision,   Acuity  of    42 

Visual    Angle     42 

Wheel  Test  in   .\stigmatism.l2S 
Yellow    Spot     38 


y^^^^- 


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